Apparatus and methods for non impact imaging and digital printing

ABSTRACT

The present disclosure presents an apparatus for pattern generation on a dielectric substrate comprising an imaging drum including a dielectric pattern receiving and retaining substrate, a plurality of electrodes underlying the pattern receiving and retaining dielectric substrate, imaging circuitry for application of voltage signals to the plurality of electrodes, an elongate charge source operative to apply a flow of charges to the dielectric substrate, thereby creating a latent image thereon, a developing unit operative to apply toner to the dielectric substrate, thereby producing a toned image according to the latent image, and a transfer unit operative to transfer the toned image to a substrate.

REFERENCE TO CO-PENDING APPLICATIONS

This application is a continuaton-in-part of U.S. Pat. application Ser.No. 08/398,621 filed Mar. 1, 1995 and assigned to the assignee of thepresent invention, which is a continuation-in-part of U.S. Pat.application Ser. No. 08/306,052, filed Sep. 14, 1994, now U.S. Pat. No.5,508,727,which is a continuation -in-part of U.S. Pat. application Ser.No. 07/944,157 filed Sep. 11, 1992, now abondoned, which is acontinuation-in-part of U.S. Pat. application Ser. No. 07/1766,691,filed Sep. 27,1991, now U.S. Pat. No. 5,289,214, which is acontinuation-in-part of U.S. Pat. application Ser. No. 07/697,166, filedMay 8, 1991, now U.S. Pat. No. 5,157,423.

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for non-impactimaging and digital printing.

BACKGROUND OF THE INVENTION

There exist in the patent literature disclosures of a great number oftechniques for non-impact printing and imaging. The most widely used ofthese techniques is electrophotography wherein an electrostatic image isoptically formed on a photoconductor, which is then developed with atoner. The toner image is transferred to a substrate and fused thereon.

An additional technique in general use is ionography, wherein anelectrostatic image is formed on a dielectric substrate by firingcharges directly onto the substrate using an imagewise ion source.

A technique for the transfer of electrostatic images from a dielectricphotoconductor onto a dielectric substrate has also been proposed inElectrophotography by R. M. Schaffert, 2nd Edition, Focal Press, London,1975 at pages 166-176 and in U.S. Pat. No. 3,055,006. This technique,known as TESI (Transfer of Electrostatic Images). employs an imagewiseoptical signal to create a charge image on a photoconductor. The chargeimage is subsequently replicated onto a dielectric substrate by applyingsingle polarity charges to a surface of the dielectric substrateopposite from that surface which faces the photoconductor.

SUMMARY OF THE INVENTION

There is thus provided in accordance with a preferred embodiment of thepresent invention imaging apparatus including a dielectric substratewith at least two generally opposite surfaces; a plurality of elongateelectrodes underlying a first surface of the dielectric substrate;imaging circuitry for application of voltage signals to the plurality ofelectrodes; a charge source operative to supply a flow of charges to asecond surface of the dielectric substrate not beyond a generally linearboundary disposed along the second surface of the dielectric substrateand transversing the plurality of elongate electrodes.

Additionally in accordance with a preferred embodiment of the presentinvention, the charge source includes an edge of an elongate shieldwhich defines the generally linear boundary.

Moreover, in accordance with yet a further embodiment of the invention,the elongate shield includes an electrostatic shield.

In yet a further embodiment of the invention, the elongate shieldincludes a physical barrier.

In further accordance with an embodiment of the present invention, thecharge source includes an AC corona.

Further in accordance with a. preferred embodiment of the presentinvention, the charge source includes a liquid which exhibits electricalconductivity; an applicator which is operative to apply the liquid tothe second surface of the dielectric substrate; a termination electrode;and a physical barrier which defines the generally linear boundary.

Alternatively, the liquid may be characterized in that it transferscharges to the second surface of the dielectric element.

In accordance with yet a further embodiment, the applicator is operativeto rinse the second surface of the dielectric element with the liquid,thereby also providing a cleaning function.

Moreover, the physical barrier may include a dielectric blade.

Furthermore, the termination electrode includes a conductive coating onone surface of the dielectric blade.

Stll further in accordance with a preferred embodiment of the presentinvention, the conductive coating is biased to a predetermined uniformpotential.

In accordance with yet a further embodiment of the present invention,apparatus further includes a developing unit operative to produce adeveloped image on the dielectric substrate.

Moreover, the apparatus may include a transfer unit operative totransfer the developed image to an output medium.

There is thus provided in accordance with a preferred embodiment of thepresent invention, a method for providing a dielectric substrate with atleast two generally opposite surfaces; providing a plurality of elongateelectrodes underlying a first surface of the dielectric substrate;applying voltage signals to the plurality of electrodes; supplying aflow of charges to a second surface of the dielectric substrate up toand not beyond a generally linear boundary disposed along the secondsurface of the dielectric substrate and transversing the plurality ofelongate electrodes.

Moreover, the step of supplying a flow of charges includes extraction ofions from an ion pool.

Yet in further accordance with the present invention, the step ofsupplying a flow of charges includes the steps of applying a liquid tothe second surface of the dielectric substrate; providing a terminationelectrode; and providing a physical barrier which defines the generallylinear boundary.

Further in accordance with the present invention, the applying of aliquid is further operative to rinse the second surface of thedielectric element with the liquid, thereby providing a cleaningfunction.

Additionally, the step of producing a developed image on the dielectricsubstrate may be included.

According to yet a further embodiment of the present invention, the stepof transfering the developed image to an output medium is included.

In accordance with yet a further embodiment of the present invention,the step of displacing the generally linear boundary relative to thedielectric substrate is included.

There is thus provided in accordance with the present invention, digitalcolor printing apparatus including an intermediate transfer medium; aplurality of imaging units disposed about the intermediate transfermedium and operating in time synchronization wherein each printing unitincludes: an imaging drum having an outer dielectric surface; aplurality of elongate electrodes underlying the outer dielectricsurface; imaging circuitry for application of voltage signals to theplurality of elongate electrodes; a charge source operative to supply aflow of charges to the outer dielectric surface up to and not beyond agenerally linear boundary disposed along the outer dielectric surface ofthe imaging drum and transversing the plurality of elongate electrodes;a developing unit operative to apply pigmented toner to the outerdielectric surface, producing toned images thereon; and a primarytransfer unit operative to transfer the toned images to the intermediatetransfer medium; and a secondary transfer unit operative to transfer thetoned images from the intermediate transfer medium to a print medium.

Moreover, each of the developing units of the plurality of imageprinting units utilizes a distinct toner color.

Furthermore, the plurality of image printing units are operative toprovide full process color.

In accordance with an embodiment of the present invention, theintermediate transfer medium includes a drum.

Alternatively, the intermediate transfer medium may include a belt.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIGS. 1A, 1B, 1C and 1D are illustrations of the application of voltageor charge on various surfaces over time in accordance with a preferredembodiment of the present invention;

FIG. 2 is an illustration of a time varying voltage signal on a firstsurface of a dielectric substrate resulting in a corresponding chargepattern being retained on an opposite surface of the dielectricsubstrate in accordance with a preferred embodiment of the presentinvention;

FIGS. 3A and 3B are respective generalized and side view illustrations,taken along the line IVB--IVB, of apparatus for applying voltage andcharges to opposite surfaces of a dielectric substrate in accordancewith another embodiment of the present invention;

FIGS. 4A and 4B are respectively graphical and pictorial illustrationsof the operational parameters of apparatus for continuous toning byoptical density modulation in accordance with a preferred embodiment ofthe invention;

FIG. 5 is a schematic illustration of an alternate embodiment of theapparatus of FIGS. 3A and 3B;

FIGS. 6A, 6B and 6C are illustrations of parameters for obtaining acontinuum of gray levels by pixel size modulation in accordance with anembodiment of the present invention;

FIGS. 7A and 7B are pictorial illustrations of side and top views,respectively, of an alternative embodiment of an alternating polaritycharge source providing a charge flow having at least one defined edge;

FIG. 7C is an alternative embodiment of the alternating polarity chargepool apparatus of FIG. 7A.

FIG. 8 is a graphical illustration of the intensity of an ion currentmeasured by a current measuring device as a function of the relativedisplacement in a sweep direction between an alternating polarity chargesource and the device in accordance with a preferred embodiment of thepresent invention;

FIGS. 9A and 9B are pictorial side and top view illustrationsrespectively, of a further alternating polarity charge source providinga charge flow having at least one defined edge;

FIGS. 10A and 10B are pictorial side and top illustrations of analternative embodiment of an alternating polarity charge sourceproviding a charge flow having at least one defined edge in accordancewith the present invention;

FIG. 11A is a pictorial illustration of a side view of apparatus forproviding an alternating polarity charge flow having at least onedefined edge in accordance with a further embodiment of the presentinvention.

FIG. 11B is an illustration of the amplitude modulation of voltageapplied to apparatus for providing an alternating polarity charge flowover time in accordance with a preferred embodiment of the presentinvention.

FIG. 11C is a pictorial illustration of a side view of apparatus forproviding an alternating polarity charge flow having at least onedefined edge in accordance with an alternate embodiment of the presentinvention.

FIG. 12A is a simplified illustration of printing apparatus constructedand operative in accordance with another preferred embodiment of thepresent invention;

FIGS. 12B and 12C are signal diagrams illustrating electrical signalsproduced by the apparatus of FIGS. 3A and 3B;

FIG. 13A is a simplified illustration of printing apparatus constructedand operative in accordance with another preferred embodiment of thepresent inventon;

FIG. 13B is an illustration of the operation of the apparatus of FIG.13A;

FIG. 14A is a generalized pictorial illustration of a digital receptorimaging drum constructed and operative in accordance with a preferredembodiment of the present inventon;

FIG. 14B is a sectional illustration of a portion of the drum of FIG.14A, taken at line B--B on FIG. 14A;

FIGS. 15A, 15B, 15C and 15D are sectional illustrations, taken alongline B of FIG. 14A, of four alternative embodiments of the imaging drumof FIG. 14A;

FIG. 16A is a partially cut away generalized pictorial illustration of apreferred embodiment of the drum of FIGS. 14A and 15C;

FIG. 16B is a partially sectional, partially pictorial illustration of afurther variation of the imaging drum of FIGS. 14A and 15C;

FIG. 16C is an enlarged illustration of a portion of the imaging drum ofFIG. 16A, at an intermediate stage of drum fabricaton;

FIG. 16D is a generalized pictorial illustration of a further variationof the imaging drum of FIGS. 14A and 15C;

FIG. 16E is a partially cut-away illustration of part of the imagingdrum of FIG. 16D;

FIGS. 17A and 17B are schematic illustrations of alternate embodimentsof the imaging electronics used in the apparatus of FIGS. 14A and 14B;

FIG. 18 is an illustration of a continuous tone pattern generated usingthe apparatus of FIG. 17A;

FIGS. 19A and 19B are block diagram illustrations of the architecture ofthe apparatus of FIG. 17A when used for producing a continuous tonepattern;

FIG. 20 is an illustration of a halftone pattern generated using theapparatus of FIG. 17A

FIGS. 21A and 21B are block diagram illustrations of the architecture ofthe apparatus of FIG. 17A when used for producing a pixel-size modulatedhalf-tone pattern; and

FIG. 22 is a block diagram illustration of the architecture of theapparatus of FIG. 17A when used for producing a super-pixel halftonepattern.

FIG. 23 is a schematic illustration of printing engine apparatusoperative in accordance with a preferred embodiment of the presentinvention.

FIG. 24 is a structural illustration of printing engine apparatusrepresenting an embodiment of the present invention.

FIG. 25 is a schematic illustration of digital color copier apparatusoperative in accordance with a preferred embodiment of the presentinvention.

FIG. 26 is a schematic illustration of print laminator apparatusoperative in accordance with a preferred embodiment of the presentinvention.

FIG. 27 is a schematic illustration of printing apparatus operative toprovide high speed one-sided or duplex printing in an embodiment of thepresent invention.

FIG. 28 is a schematic illustration of printing apparatus operative toprovide high speed one-sided or duplex printing in accordance with analternative embodiment of the present invention.

FIG. 29 is a schematic illustration of printing apparatus operative toprovide high speed one-sided or duplex printing in accordance with afurther embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is now made to FIGS. 1A-1D, which illustrate the operation ofthe present invention. FIGS. 1A illustrates an arbitrary voltage at atypical point location on a first surface of a dielectric substrate asit varies over time. The voltage may be applied to the typical pointlocation by means of a conductive backing associated with the firstsurface of the dielectric substrate in touching or capacitiverelationship therewith. The conductive backing may be a separateconductor in close proximity to, or alternatively a permanent coating orlayer formed on, the first surface of the dielectric substrate.

FIG. 1B illustrates, on the same time scale as in FIG. 1A, theapplication of a flow of charges to a second surface of the dielectricsubstrate, which is opposite to and generally uniformly spaced from thefirst surface, to temporarily neutralize the effect on the secondsurface of the voltage applied to the first surface. Followingapplication of such charges the second surface retains a chargecorresponding to the voltage which was applied to the first surface, atthe time T2 that the application of such charges ceased, but of anopposite polarity thereto.

According to a preferred embodiment of the invention, the flow ofcharges comprises an alternating polarity charge flow to a secondsurface of the dielectric substrate which is opposite to and generallyuniformly spaced from the first surface. It is preferred that the timevariation of the voltage applied to any given location on the firstsurface be sufficiently small such that at least during an end portionof the duration of the alternating polarity charge flow at suchlocation, the voltage variation is essentially zero.

The alternating polarity charge flow at each location is represented inFIGS. 1B by a stack of positive and negative charges. The beginning andend of the duration of the application of the alternating polaritycharge flow at each location are indicated in FIG. 1B respectively as T1and T2.

FIG. 1C illustrates the apparent surface voltage on the second surfaceof the dielectric substrate. It is noted that this voltage tracks thevoltage on the first surface until the charge flow begins at time T1.Upon termination of the charge flow at time T2, the apparent surfacevoltage of the second surface is very nearly zero. If, thereafter, thevoltage on the first surface is brought to zero, as indicated in FIGS.1A, the apparent surface voltage on the second surface becomesapproximately the negative of the voltage on the first surface at timeT2, due to charge retention on the second surface, as indicated in FIG.1C.

FIG. 1D is an expanded time scale illustration corresponding to FIG. 1Cand illustrating with greater particularity the effect of one possibleapplication of an alternating charge flow to the second surface, whichresults in a reduction in the apparent surface voltage on the secondsurface from the voltage at T1 to very nearly zero at T2.

Referring now to FIG. 2, there is shown schematically an arbitraryvoltage signal provided on a dielectric substrate 10 at a first surface12 thereof, which is preferably backed with a conductive backing 14 towhich is coupled a time-variable voltage source 16. FIG. 2 alsoillustrates in one dimension, the corresponding spatial charge pattern,of opposite polarity to the corresponding voltage signal, which isretained on a second surface 18 of the dielectric substrate inaccordance with the dynamic charge retention of the present invention,by application of a flow of charges to the second surface which isoperative to temporarily neutralize the effect on the second surface ofthe voltage applied to the first surface 12. The application of the flowof charges is preferably provided by an alternating polarity chargesource (APCS) 20, capable of achieving a spatial edge accuracyconsistent with the desired resolution, which may be of the typedescribed hereinbelow.

The charge source 20 is preferably moved at a velocity v along thesecond surface 18 of the dielectric substrate 10, indicated by an arrow22.

Reference is now made to FIGS. 3A and 3B which illustrate apparatus forapplying voltage and charges to opposite surfaces of a dielectricsubstrate 42 in accordance with another embodiment of the presentinvention.

In this embodiment, an elongate alternating polarity charge source(EAPCS) 46, such as that described herein and being capable of achievinga spatial edge accuracy consistent with the desired resolution isscanned in one dimension, perpendicular to its longitudinal axis, alonga second surface 48 of substrate 42, by means of a linear drivemechanism including a worm screw 50 cooperating with a screw rider 52,fixed to source 46. An electric motor 54 drives the worm screw 50 inresponse to the outputs of a commercially available synchronized driver56. A host computer (not shown) provides positioning instructions via amultiplexer 58 to driver 56.

In this embodiment, a multisectional conductive backing layer 60,typically comprising a plurality of elongate electrodes 44, isassociated with the first surface of the dielectric substrate 42. Eachelectrode 44 is provided with an information content modulated timevarying voltage via a corresponding driver 62, in response to controlsignals received from the host computer via multiplexer 58.

It may be appreciated that in the embodiment of FIGS. 3A and 3B a chargepattern is written on the second surface by information contentmodulation of voltages applied simultaneously to the different regionsof the first surface of the dielectric substrate 42 via electrodes 44 insynchronism with the one dimensional scanning motion of source 46.

It is appreciated that a desired two-dimensional spatial resolution maybe achieved by adjusting appropriate parameters. In a first dimension,the parameters to be adjusted include the width of the elongateelectrodes 44, the width of a gap 45 between adjacent electrodes 44, thethickness of the dielectric layer 42 and the dielectric constant of thedielectric layer 42. In a second dimension, the parameters include theedge definition of the EAPCS 46. The gaps 45 should be filled withelectrically insulative dielectric material having high dielectricstrength. Alternatively, gaps 45 may be filled with highly resistivematerial.

It is further appreciated that the dynamic charge retention techniquesdescribed hereinabove allow an uninterrupted line of uniform width assmall as one pixel to be achieved. The advantages of these techniquesinclude the replacement of dot pixels thus eliminating holes betweenpixels. Accordingly, intentional overlap of pixels is not necessary.

Mult-sectional conductive backing layer 60 may comprise a uniformconductive film from which a plurality of electrodes are created usingan etching technique such as laser etching, chemical etching, or ionetching. Alternatvely the multi-sectional conductive backing layer maycomprise a grid comprising a weave of straight conductive wires in onedimension and insulating curved wires in a second dimension. This typeof grid is available from Carbotex of Grutlistrasse 68, Zurich,Switzerland. Alternatively, the multi-sectional conductive backing layer60 may be produced by electroforming or winding techniques.

Reference is now made to FIGS. 4A and 4B which illustrate operationalparameters for continuous toning apparatus in accordance with apreferred embodiment of the present invention.

It is appreciated that certain toners are characterized by the propertythat the optical density of a toned area can be controlled by thedevelopment voltage. Some liquid toners and certain dry toners areexamples of this type of toner.

It is also appreciated that the dynamic charge retention techniquesdescribed hereinabove are capable of writing continuous voltage levelsthus enabling the generation of one-pass monochrome toned images ofcontinuous optical densities, when toners of the type describedhereinabove are used.

In accordance with a preferred embodiment of the present invention,continuous color printing using standard subtractive colors and opticaldensity modulation can be achieved in accordance with the dynamic chargeretention writing techniques described herein and standard multi-passprinting techniques.

FIG. 4A graphically demonstrates hypothetical optical densities of fourbasic printing colors (CMYK) typically used with subtractive colorprinting systems as a function of the development voltage of each of thetoner colors.

FIG. 4B illustrates a single area of a color print 345 to which fourbasic printing colors (CMYK) have been sequentially transferred inaccordance with standard multi-pass printing techniques.

The specific optical density of each color across any area may becontrolled by writing (in accordance with the writing techniquesdescribed hereinabove in association with FIGS. 3A and 3B) at thecorresponding area a voltage level which corresponds to the desiredoptical density for that color.

It is appreciated that the subtractive combination of the opticaldensities of each of the four basic printing colors over an area resultsin a color having any of a continuum of color shades.

It is also appreciated that a color shade may be uniformly distributedwithin the borders of an entire area. Therefore, the specific colorshade desired is achieved at the level of one pixel and not as a resultof the combination of several pixels.

This embodiment offers continuous control over color levels providinghigh quality color prints.

Reference is now made to FIG. 5 which illustrates a schematicrepresentation of imaging electronics for use in the apparatus of FIGS.3A and 3B for writing continuous as well as half-tone levels.

A clock input 352 is pulsed in coordination with input data causing ashift register 354 to sequentially address each of a plurality ofconductive electrodes 356. The conductive electrodes 356 may correspondto conductive electrodes 44 in FIGS. 3A and 3B. The electrode beingaddressed at any given time receives a voltage level from imagingelectronics 358 and is charged to that voltage level. Typically, imagingelectronics 358 comprises a digital to analog high voltage converter.

Associated with each electrode 356 is a capacitor 360 which retains thegiven voltage level until the electrode is subsequently addressed andreceives a new voltage level.

It is appreciated that the dynamic charge retention techniques of thepresent invention allow charges of either polarity to be written to thesubstrate. It is further appreciated that one individual charge patterncan contain charges of both polarities. The number of voltage levelsachievable in accordance with this embodiment is not dependent on theprint head. Instead, the number of voltage levels is determined by theimaging electronics 358.

Reference is now made to FIGS. 6A -6C which illustrate a method forobtaining a continuum of monochrome gray levels by providing pixel sizemodulation in accordance with an embodiment of the present invention.

FIG. 6A graphically represents monochromatic gray levels that can beachieved over an area as a function of the fractions of each of thepixels comprising that area that are toned. Pixels are charged and tonedin accordance with the writing techniques described hereinabove,particularly with reference to FIGS. 3A and 3B, and standard toningtechniques.

The dimension of a pixel in the direction of the sweep of the chargesource may be represented by L. The dimension of a pixel in the seconddirection is determined by the width of the conductive electrodeassociated with that pixel. (a/L) is the toning fraction, where a is afunction of the velocity of the charge source multiplied by theeffective time duration during which the bias voltage on the conductiveelectrode changes during the writing of a single pixel.

Curve 470 illustrates one possible representation of gray levels thatcan be achieved by controlling the toning fraction of each pixel. Inthis representation, the voltage biasing the conductive electrode iszero at the beginning and end of a sweep of the pixel by a chargesource. During the sweep, the voltage biasing the conductive electrodeis raised to a high level for a time duration determined by the desiredtoning fraction.

Curve 472 illustrates an alternate representation of gray levels thatcan be achieved by controlling the toning fraction of each pixel. Inthis representation, the voltage biasing the conductive electrode is ata high level at the beginning and end of a sweep of the pixel by acharge source. During the sweep, the voltage biasing the conductiveelectrode is reduced to zero for a time duration in accordance with thedesired toning fraction.

It is appreciated that a continuum of monochrome gray scales may beachieved for an area by selecting the appropriate voltages for thebeginning and end of the sweep of each of the pixel locations comprisingthat area (following curve 470 or curve 472 depending upon the desiredgrey level).

FIGS. 6B and 6C illustrate close-up views of the fractionally tonedimage of the pixels of two adjacent conductive electrodes (not shown)where the pixels of one electrode have been phase shifted with respectto the pixels of the second electrode.

Two different gray scales are shown (FIG. 6B and FIG. 6C).

It is appreciated that the phase shift with respect to adjacentelectrodes enabled by pixel size modulation techniques describedhereinabove allows half tone gray scales with high spatial frequency ofpixel arrangements to be achieved.

It is further appreciated that continuous color shades, via half tone,can be achieved using pixel size modulation techniques to vary thetoning fraction of each pixel during each of four monochrome passescarried out during standard multi-pass subtractive color printing.

It is appreciated that the dynamic charge retention techniques describedhereinabove, particularly with reference to FIGS. 3A and 3B, provide amethod for generating a charge pattern that contains charges of bothpolarities. It is understood that there exist toners which developpositive charge images and similarly there are other toners whichdevelop negative charge images.

Therefore by using two different color toners which develop oppositepolarity charge images, a two-color image may be produced in a singlepass. The two-color image may contain any of a continuum of shades ofthe two colors, in accordance with the techniques for continuous toningdescribed hereinabove, or alternatively, in accordance with thetechniques for pixel size modulation half-tones described hereinabove.One possible application for this technique is in the generation of"highlight" images.

Reference is now made to FIGS. 7A-7B which are pictorial illustrationsof side and top views, respectively, of an alternating polarity chargesource providing a charge flow having at least one defined edge inaccordance with an embodiment of the present invention.

Alternating polarity charge pool apparatus 502 comprises a non-imagewisesource of ions. Apparatus 502 may also comprise an elongate conductor504 coated with a dielectric layer 506 and a transversely orientedscreen electrode 508 contacting or closely spaced from thedielectric-coated conductor and coiled about an inner dielectric supportstructure 507 as described in U.S. Pat. No. 4,409,604 assigned toDennison Manufacturing Company of Framingham, Mass., USA .

Typically inner dielectric support structure 507 is rod shaped.Alternately, inner dielectric support structure 507 may be of anysuitable shape such as that illustrated in FIG. 7C below.

In accordance with a particular configuration of apparatus 502,transversely oriented screen electrode 508 is coiled around elongateconductor 504. When apparatus 502 is operational, a pool of positive andnegative ions is continuously generated in the air space immediatelysurrounding the dielectric-coated conductor 504 at the regions inbetween the locations where the coiled conductor crosses over thedielectric-coated conductor.

An elongate electrostatic shield 510, typically comprising a conductivematerial, may be configured as illustrated to partially enclose chargepool apparatus 502.

It is appreciated that the charge source of FIGS. 7A-7B is capable ofproviding an ion beam with at least one edge sharply defined. It isappreciated that charge pool apparatus 502 alone does not provide anelongate ion beam with a sharply defined edge.

It is further appreciated that charge pool apparatus 502 may serve as anelongate alternating polarity charge (EAPCS) source describedhereinabove in accordance with dynamic charge retention techniquesdescribed hereinabove. This configuration is presented to offer anexample of possible configurations for the EAPCS and is not intended tobe limiting.

An excitation voltage is applied to elongate conductor 504 eithercontinuously or in amplitude modulated bursts of any suitablepredetermined wave form. The excitation voltage is typically a high ACvoltage with an amplitude of 2000V pp and has a frequency in the rangebetween several hundred kHz to several MHz.

Typically, elongate electrostatic shield 510 and transversely orientedscreen electrode 508 are grounded and the dominant force in chargeextraction is the ASV. As the ASV decreases, charge extraction isreduced accordingly until neutralization of the ASV.

A dielectric surface 512, typically of the type described hereinaboveparticularly in accordance with FIGS. 3A-3B, sweeps relative to thecharge source. Alternatively, dielectric surface 512 may be an outerdielectric surface of a digital input electrostatic (D/E) imaging drumas described in one or more of applicant's co-pending patents and patentapplications including U.S. Pat. Nos. 5,289,214 and 5,157,423, thedisclosure of which are hereby incorporated by reference. Alternately,any other suitable dielectric surface on which charges are to beaccumulated may be used.

Preferably, the gap between dielectric surface 512 and shield 510 isabout 100-300 microns. Typically, dielectric surface 512 comprises aplurality of regions each having associated therewith an apparentsurface voltage (ASV). During the sweep, these regions of the dielectricsurface are brought into their maximum propinquity with the charge pool.The ASV of each region creates an electric field between dielectricsurface 512 and the charge pool.

Electrostatic shield 510 serves to tailor the electric field created.The ASV at regions on the dielectric surface which have directunshielded access to the charge pool causes ions of the appropriatepolarity to be extracted until the ASV at that region is neutralized.The ASV at regions on dielectric surface 512 which are shielded from thecharge pool are not involved in ion extraction. When a region on thedielectric surface is in a location which is not shielded from thecharge pool, the region may accumulate charges in accordance with thetechniques described hereinabove. When the region is moved to a shieldedlocation, additional charge will not accumulate on that region.

According to an alternate mode of operation, elongate electrostaticshield 510 and transversely oriented screen electrode 508 receive ascreen AC voltage, instead of being grounded. The screen AC voltagecomprises a phase shifted high amplitude modulated AC voltage at thesame frequency as the excitation voltage applied to elongate conductor504 to obtain the effect illustrated by the graph of FIGS. 1D. Theamplitude of the screen AC voltage is on the same order of magnitude asthe maximum ASV, which is selected in accordance with charge imageparameters. In this case, charge extraction is a combined function ofthe ASV and the screen AC voltage. The phase shift, between the screenAC voltage and the excitation voltage supplied to elongate conductor504, determines the efficiency of charge extraction from the charge poolby the screen AC voltage. This provides the advantage of acceleratedneutralization of the ASV. As the ASV is neutralized, the amplitude ofthe screen AC voltage is gradually decreased to zero, so that the finalcharge level retained corresponds only to the ASV that was present onthe dielectric surface 512 prior to activation of the charge poolapparatus 502. It is appreciated that amplitude modulation techniquesmay be used in accordance with any of the embodiments of FIGS. 7A-11A.

It is appreciated that the extracted ions typically form a beam with atleast one sharply defined edge. In accordance with the techniquesdescribed in accordance with this embodiment of the invention,preconditioning of the dielectric surface to be charged is notnecessary. Therefore, the width of the charge beam is not significant aslong as one edge is sharply defined. It is also appreciated that chargesfrom the opposite, undefined edge of the ion beam do not affect thefinal amount of charge that is retained at a region. When the regionmoves past the sharply defined edge, the appropriate amount of negativeor positive charges is retained to balance the effect of the conductivebacking potential regardless of any stray charges that may haveaccumulated at that region from the opposite, undefined edge. The edgeresolution achievable in the direction of the sweep may be a function ofthe intensity of the ion beam and the sharpness of the beam edge.

According to an alternative embodiment, an elongate grounded shieldgenerally enclosing charge pool apparatus 502, except for an elongateopening, may be used to provide an ion beam with two defined edges.

Reference is now made to FIG. 7C which is a pictorial illustration of aside view of an alternative embodiment of alternating polarity chargepool apparatus 502 of FIG. 7A in accordance with an alternativeembodiment of the present invention.

Alternating polarity charge pool apparatus 511, comprises anon-imagewise source of ions. Apparatus 511 may further comprise anelongate conductor 513 coated with a dielectric layer 515 and atransversely oriented screen electrode 517 contacting or closely spacedfrom the dielectric-coated conductor and coiled about an innerdielectric support structure 519 as described in U.S. Pat. No.4,409,604assigned to Dennison Manufacturing Company of Framingham, Mass., USA.

In the embodiment of apparatus 511 of FIG. 7C, inner dielectric supportstructure 519 is shaped so as to provide greater exposure of elongateconductor 513 to screen electrode 517. This allows increased iongeneration without compromising the mechanical strength of alternatingpolarity charge pool apparatus 511.

Reference is now made to FIG. 8 which is a graphical illustration of theintensity of an ion current measured by a current measuring device as afunction of the relative displacement in a sweep direction between analternating polarity charge source and the device in accordance with apreferred embodiment of the present invention;

For the measurements illustrated herein, a conductive current probe witha constant bias voltage of 400 V (providing an ASV of 400 V) was used tomonitor the steady-state ion beam.

Curve 520 illustrates a typical ion beam profile for the case whereapparatus 502 (FIGS. 7A and 7B above) does not comprise an electrostaticshield.

Curve 522 illustrates a typical ion beam profile for the case whereapparatus 502 (FIGS. 7A and 7B above) comprises an electrostatic shield,typically of the type described hereinabove and indicated by referencenumber 510.

The two sets of measurements were carried out under the same conditions.In particular, the distance between the probe and the charge poolremained unchanged. Curve 522 illustrates the region in which the beamhas a sharp edge. The shaded area represents the uncertainty of themeasurement due to the accuracy of the measuring device. The edge of theion beam (illustrated by curve 522) using the shield apparatus issharply defined. By contrast the edge of the ion beam (illustrated bycurve 520) in the non-shielded apparatus is not sharply defined.

It is appreciated that curves 520 and 522 illustrate ion beams understatic conditions where the ASV does not change. Under dynamicconditions, when the ASV changes with time, the ion beam of theunshielded apparatus may exhibit a high degree of blooming. It isappreciated that under the same dynamic conditions, the ion beam of theshielded apparatus will not exhibit such blooming at the edge.

Reference is now made to FIGS. 9A-9B which are pictorial respective sideand top view illustrations of apparatus 530 for providing an alternatingpolarity charge source having at least one defined edge.

Apparatus 530 comprises alternating polarity charge pool apparatus 532which includes a non-imagewise source of ions, comprising a high voltageelectrode 534 coupled to a high voltage AC source 535 and two typicallygrounded screen electrodes 536 and 538. Alternately screen electrodes536 and 538 may receive an amplitude modulated voltage in accordancewith the techniques described herein.

Screen electrodes 536 and 538 are separated from high voltage electrode534 by a dielectric layer 540. A space 542 is defined between screenelectrodes 536 and 538, providing a slot in which an ion pool may begenerated. Alternatively screen electrodes 536 and 538 may be replacedby a single slotted electrode. The electrode arrangement may be asdescribed in U.S. Pat. No. 4,155,093 assigned to Dennison ManufacturingCompany of Framingham, Mass., USA.

It is appreciated that alternating polarity charge pool apparatus 532does not itself provide an elongate ion beam with a sharply definededge.

In order to achieve an ion beam which has at least one edge defined inaccordance with the techniques described hereinabove particularly withrespect to FIGS. 7A-7C, apparatus 530 further comprises an elongateelectrostatic shield 544. Shield 544, typically comprising a preferablyconductive material, is located with some spacing relative to chargepool apparatus 532. Typically shield 544 is grounded. Alternately,shield 544 may receive an amplitude modulated voltage in accordance withthe techniques described herein. Electrostatic shield 544 may beconfigured as illustrated whereby charge pool apparatus 532 is partiallyobscured by shield 544.

It is appreciated that apparatus 530 may serve as an elongatealternating polarity charge source in the dynamic charge retentiontechniques described hereinabove. It is noted that the configurationpresented here is a further example of a possible configuration for thecharge source and is not intended to be limiting.

Reference is now made to FIGS. 10A and 10B which are pictorialillustrations of apparatus 550 for providing an alternating polaritycharge flow having at least one defined edge.

Apparatus 550 comprises alternating charge pool generating apparatus552, a casing 556 and an elongate electrostatic shield 560. Apparatus552 preferably comprises one or more corona wires 554 some or all ofwhich may be dielectrically coated. Alternatively, corona wires 554 maynot be dielectrically coated. Each corona wire 554 is operative toreceive a high AC voltage. All of the wires 554 may be biased by thesame AC source (not shown). Alternatively, corona wires 554 may bebiased by AC sources having different amplitudes. Each corona wire 554may receive a different AC voltage.

Corona wires 554 are confined by an isolating casing 556, typicallyformed of a dielectric material. Casing 556 typically contains anelongate screen electrode 558 which is partially open and wherein an ionpool is created. Screen electrode 558 and electrostatic shield 560 aretypically grounded. Alternately, when using amplitude modulationtechniques as described herein, screen electrode 558 and electrostaticshield 560 receive a high amplitude modulated AC voltage.

The open area of screen 558 may comprise a gridlike area as shown.Alternatively, the open area may comprise at least one elongate slot.

According to an alternate embodiment of apparatus 552, casing 556 mayfurther comprise an inlet through which conditioned air may flow ontocorona wires 554. In this embodiment the intensity of the ion poolcreated at screen 558 may be increased.

In order to achieve an ion beam which has at least one edge as describedhereinabove particularly with reference to FIGS. 7A-7B, elongateelectrostatic shield 560 typically comprising a grounded conductivematerial, is spaced relative to charge pool apparatus 552. Electrostaticshield 560 may be configured as illustrated so that charge poolapparatus 552 is partially obscured.

It is appreciated that apparatus 550 may serve as an elongatealternating polarity charge (EAPCS) source described hereinabove inaccordance with the dynamic charge retention techniques describedhereinabove. This configuration is an example of a possibleconfiguration for the EAPCS and is not intended to be limiting.

Reference is now made to FIG. 11A which is a pictorial illustration of aside view of apparatus for providing an alternating polarity charge flowhaving at least one defined edge in accordance with a further embodimentof the present invention.

Apparatus 598 comprises alternating charge pool apparatus 600 andbarrier 602 with elongate edge 604. Alternating polarity charge poolapparatus 600 may be of the type described hereinabove with reference toFIGS. 7A -10B, with particular reference to FIG. 7C.

A dielectric surface 512, typically of the type described hereinaboveparticularly in accordance with FIGS. 3A -3B, sweeps relative toalternating polarity charge flow apparatus 598. Preferably, barrier 602is placed in contacting proximity of dielectric surface 512. It isappreciated that edge 604 of barrier 602 serves as an electrostaticshield.

Typically, dielectric surface 512 comprises a plurality of regions eachhaving associated therewith an apparent surface voltage (ASV). Duringthe sweep, these regions of the dielectric surface are brought intotheir maximum propinquity with charge pool 606.

When a screen electrode (not shown) of alternating charge pool apparatus600 is grounded, as described hereinabove, with reference to FIGS. 7A-10B, the ASV of each region creates an electric field betweendielectric surface 512 and charge pool 606 causing ions to be extractedand deposited on the dielectric substrate 512, neutralizing the ASV.

Alternately, modified amplitude modulation techniques, with itsassociated benefits, may be used, whereby a high amplitude modulated ACvoltage is applied to the screen electrode (not shown) of alternatingcharge pool apparatus 600.

Charge is supplied up to and not beyond edge 604 of barrier 602. Thus,after an area of dielectric surface 512 has swept over edge 604 ofbarrier 602, additional charge will not reach that area due to thegenerally linear physical boundary created by contact between elongateedge 604 of barrier 602 and dielectric surface 512. Thus, the density ofretained charge on a given region corresponds to the ASV of that regionas dielectric surface 512 sweeps over edge 604.

Barrier 602 may typically be a blade made from an elastomeric dielectricmaterial. The precise material is selected so as to be free oftriboelectricity when brought into contact with dielectric surface 512,in order to prevent interference with charge retained on dielectricsurface 512.

It is further appreciated that apparatus 598 may serve as an elongatealternating polarity charge source (EAPCS) described hereinabove inaccordance with dynamic charge retention techniques describedhereinabove. This configuration is presented to offer an example ofpossible configurations for the EAPCS and is not intended to belimiting.

Reference is now made to FIG. 11B which shows, as a function of time,the amplitude modulated AC voltage applied to the screen electrode (notshown) of apparatus 600 of FIG. 11A, in accordance with the amplitudemodulation techniques described herein.

Reference is now made to FIG. 11C which is a pictorial illustration of aside view of apparatus for providing an alternating polarity charge flowhaving at least one defined edge in accordance with a further embodimentof the present invention.

Alternating polarity charge flow apparatus 610 is operative to supply anedge-defined flow of charges of either polarity to a dielectric surface612.

Typically, dielectric surface 612 is capacitively associated with aplurality of electrodes (not shown) which receive informaton-bearingvoltage signals from imaging electronics and which create a plurality ofregions thereon, each having associated therewith an apparent surfacevoltage (ASV).

Dielectric surface 612 may be of the type described hereinabove withparticular reference to FIGS. 3A -3B. Alternatively, dielectric surface612 may be an outer dielectric surface of a digital input electrostatic(DIE) imaging drum as described in one or more of applicants co-pendingpatents and patent applications including U.S. Pat. Nos. 5,289,214 and5,157,423. Alternately, any other suitable dielectric surface on whichcharges are to be accumulated may be used.

Alternating polarity charge flow apparatus 610 comprises a liquidapplicator 613, a shield 614, and an elongate termination electrode 618.Shield 614 typically extends in a direction perpendicular to a sweepdirection of dielectric substrate 612.

Liquid applicator 613 is operative to apply a liquid 620 to dielectricsurface 612. Liquid 620 may be electrically conductive. Alternatively,liquid 620 may be a non-conductive liquid which exhibits electricalconductivity when subject to certain stimuli.

Liquid 620 preferably contacts but does not wet dielectric surface 612.For example, if dielectric surface 612 is hydrophobic, then water may bean appropriate conductive liquid. In accordance with the specificproperties and viscosity of liquid 620, liquid applicator 613 may applyliquid 620 to dielectric surface 612 using contact means (roller,sponge) or non-contact means (spray jets or non-contacting rollers), ora combination thereof.

Shield 614 may comprise any suitable means for establishing a generallylinear boundary with at least one well-defined elongate edge ondielectric surface 612. Beyond the well-defined elongate edge, liquid620 contacting dielectric surface 612 is substantially not present,leaving a generally dry surface with charges retained thereon.

Shield 614 may be an elastomeric dielectric blade which is placed incontact with dielectric surface 612 to create a barrier thereon. Theprecise material may be selected so as to be free of triboelectricitywhen brought into contact with dielectric surface 612, in order toprevent interference with charge retained thereon.

Elongate termination electrode 618 is typically a conductive elementhaving a predetermined uniform potential. Preferably, elongatetermination electrode 618 is grounded. It is desirable for elongatetermination electrode 618 to be positioned near shield 614 in order tocreate ionic conduction in liquid 620 between elongate terminationelectrode 618 and dielectric surface 612 along shield 614. Typically,the gap between elongate termination electrode 618 and dielectricsurface 612 along shield 614 is between 50 -1,000 microns.

Elongate termination electrode 618 may be formed by coating a surface ofshield 614 with a conductive material. Alternately, elongate terminationelectrode 618 may be an independent element.

Charged images are created on dielectric surface 612 in the followingmanner.

Dielectric surface 612 sweeps relative to alternating polarity chargeflow apparatus 610. Liquid 620 is applied to dielectric surface 612using liquid applicator 613 as described hereinabove, creating aliquid-dielectric interface.

The ASV of each region of dielectric surface 612 creates an electricfield between dielectric substrate 612 and elongate terminationelectrode 618. Elongate termination electrode 618 serves to intensifythe electric fields stemming from the ASV of each region of dielectricsurface 612. The electric fields cause ionic conduction in liquid 620.

In accordance with the ASV at any given time, charges of either polarityflow from liquid 620 to the liquid-dielectric interface and are retainedon dielectric surface 612. Charge transfer from a conductive liquid to adielectric surface is known in the art and is described in U.S. Pat. No.2,904,431 to A. J. Moncrieff Yeates, U.S. Pat. No. 2,987,660 to L. E.Walkup and U.S. Pat. No. 3,579,332 to P. W. Chudleigh.

As the ASV at a region of dielectric surface 612 is neutralized, thetransfer of charges from liquid 620 to that region diminishes.

Shield 614 provides an elongate generally linear boundary beyond whichliquid 620 on dielectric surface 612 does not pass. Thus, regions ofdielectric surface 612 which have swept over the boundary established byshield 614 no longer bear liquid 620. Accordingly, no further charge issupplied to those regions of dielectric surface 612. The density ofretained charge at a given region corresponds to the ASV of that regionas dielectric substrate 612 sweeps over the well-defined edge boundarycreated by shield 614.

It is appreciated that alternating polarity charge flow apparatus 610may serve as an elongate alternating polarity charge (EAPCS) sourcedescribed hereinabove in accordance with a modified version of dynamiccharge retention techniques described herein. It is appreciated thatalternating polarity charge flow apparatus 610 may provide liquid 620continuously to dielectric surface 612. Thus, when an alternatingpolarity charge flow apparatus such as the one described herein in FIG.11C is used as an EAPCS in dynamic charge retention techniques, it isdesirable that the voltages applid to conductive backing (not shown) ofdielectric surface 612 are continuously refreshed.

This embodiment is presented to offer an example of possible embodimentsof the EAPCS and is not intended to be limiting.

Charge patterns retained on dielectric substrate 612 in accordance withthe techniques and apparatus described herein may be developed using dryor liquid toner to establish a visible image thereon. Developing may becarried out using developing apparatus (not shown) as is known in theart and as described hereinbelow.

In accordance with an embodiment of the present invention, liquid 620may include one or more solvents which are operative to assist incleaning and/or rinsing of dielectric substrate 612. This isparticularly effective when charge image created on dielectric substrate612 is developed using a liquid toner as is known in the art and as isdescribed hereinbelow.

It is appreciated that in accordance with this embodiment, deposition ofcharge and cleaning of residues from dielectric surface 612 may becarried out simultaneously.

Reference is now made to FIGS. 12A-12C which illustrate a system forwriting and developing electrostatic images in accordance with anotherembodiment of the present invention. The illustrated embodiment employsa drum 700 having a dielectric layer 702 on its outer surface, which isrotated in a direction indicated by arrow 704.

A plurality of conducting electrodes (not shown) are embedded in thedielectric layer 702 and extend over the periphery of the outer surfaceof the drum. Electronic circuitry 706 may be mounted interiorly of theouter surface of the drum and of layer 702, whereby each electrode maybe connected to a driver forming part of the circuitry 706.

Associated with drum 700 and more particularly with dielectric layer 702is a charge source 708, preferably of the type shown in any of FIGS.7A-11A. A magnetic brush developer unit 710 and a cleaning unit 716 areoperatively associated with drum 700 in a conventional manner.

The apparatus of FIGS. 12A-12C is particularly characterized in thatcharge image generation and developing may take place simultaneously atdifferent regions of the drum. This is achieved by operating the chargesource 708 discontinuously, in a series of bursts, as indicated in FIG.12B.

During each burst of the operation of charge source 708, all of theelectrodes embedded in dielectric layer 702 receive appropriate voltagesrepresenting a single raster line of an image to be printed. Immediatelyfollowing operation of the charge source 708, i.e. in between the burstsshown in FIG. 12B, the drivers in circuitry 706 supply to each suchelectrode a voltage which is equal and opposite to the voltage appliedthereto during the operation of charge source 708.

FIG. 12C shows the voltage on a given electrode both during the burstsof operation of the charge source 708 and in between the bursts, inresponse to operation of the drivers in circuitry 706.

The result of these operations is that in each electrode, an electricalsignal is generated which is composed of high Fourier frequencycomponents and a zero DC component. The elimination of the DC componenteliminates spurious operation of the magnetic brush developer unit 710which would otherwise occur.

It is further noted that when the developer unit 710 employsdual-component toners the high Fourier frequency components in thesignal also do not result in spurious toner deposition by the developerunit 710.

In this way, the signals present on the electrodes during development bydeveloper unit 710 do not interfere with the desired development of thelatent image on the dielectric layer 702, but operate only for desiredlatent image generation.

The toned image produced by developer unit 710 is transferred to anoutput substrate, typically paper, by a transfer unit 712 and fixed tothe output substrate by a fixing unit 714 using standard toner fixingtechniques. Any residual toner on the outer surface of the drum 700 isremoved by the cleaning unit 716 using standard techniques.

Reference is now made to FIGS. 13A and 13B which illustrate a system forwriting electrostatic images in accordance with yet another embodimentof the present invention. The illustrated embodiment employs timemultiplexing of the output of a limited number of drivers to a pluralityof different arrays of individual charge sources, such as shown in FIGS.3A and 3B.

As seen in FIGS. 13A and 13B, a charge source array assembly 800includes a plurality of individual charge sources, which are indicatedby the letters a-h and are seen to be arranged in staggered, mutuallypartially overlapping relationship with a substrate 802 bearing aplurality of electrodes 804. The individual charge sources receivesignal inputs from a time multiplexer 806 which in turn receives signalinputs from drivers (not shown). The individual charge sources a-h maybe of the type illustrated in any of FIGS. 7A-11C.

In accordance with this embodiment, electrodes 804 may be connected inprimary groups which are then subdivided into secondary groups. Thenumber of primary groups is a function of the number of charge sourceline arrays in charge source array assembly 800. For example, in FIG.13A, two charge source line arrays are shown, so two primary groups ofelectrodes 804 may be used.

The secondary grouping of each primary group of electrodes 804corresponds to the number of individual charge sources making up asingle line array. For example, in FIG. 13A, there are four individualcharge sources per line array, therefore each primary group ofelectrodes 804 is subdivided into four secondary groups. It isappreciated that the electrodes 804 from each secondary group in aprimary group are connected in parallel to a shared set of high voltageimaging electronics. By connecting electrodes 804 in secondary groups,the number of outputs from, and therefore the volume of, high voltageimaging electronics is reduced.

Electrostatic charge patterns are written by sequentially activatingeach charge source of a first line array of charge source array assembly800. During activation of a charge source, voltage information issupplied to electrodes 804 located beneath the active charge source.Voltage signals supplied to each electrode 804 are also supplied to allother electrodes 804 in the same subgroup. However, only electrodes 804underlying the active charge source will retain charge in accordancewith the dynamic charge retention techniques described herein.

Charge sources in a single array are sequentially activated to write acharge image over the dielectric substrate associated with electrodes804 of an entire primary group. When substrate 802 moves relative tocharge source assembly 800, the activation and charge writing cycle isrepeated for the second primary group and its associated line array ofcharge sources.

It is appreciated that charge sources of charge source array assembly800 are arranged in an overlapping fashion. Furthermore, there existsoverlap between primary groups of electrodes 804. Thus, the need for ahigh level of registration between charge source array assembly 800 andsubstrate 802 is eliminated.

Reference is now made to FIGS. 14A and 14B which illustrate a digitalreceptor imaging drum suitable for use in a variety of imagingapplications including those described hereinabove and hereinbelow andin the copending applications referred to hereinabove, the descriptionof which is hereby incorporated by reference. FIG. 14A shows a drum 820on whose outer surface 822 latent electrostatic images may be generatedin accordance with the dynamic charge retention image writing techniquesdescribed hereinabove and hereinbelow.

The outer surface 822 includes portions which are imaging regions andother portions which are not imaging regions. Images are created at theimaging regions using an edge-defined alternating polarity charge source(not shown), and signals supplied to a conductive backing forming partof the drum. The charge source may be any suitable one of the chargesources described hereinabove, and particularly those illustrated in anyof FIGS. 7A-11C. The conductive backing preferably comprises a pluralityof electrodes.

Alternately, drum 820 may be used for image reading in accordance withtechniques described hereinbelow and in Applicant's co-pendingapplications.

FIG. 14B shows a cross section of an imaging region of the drum. Thedrum comprises an outer dielectric imaging layer 824 which may extendover both the imaging regions and the non-imaging regions. At imagingregions, a conductive backing 826 is associated with the outerdielectric imaging layer 824. The conductive backing overlies an innerdielectric layer 828. Imaging electronics (not shown) are associatedwith the electrodes of the conductive backing 826.

The outer dielectric imaging layer 824 can be made of any material thatis suitable for use with the conductive backing 826. A suitable materialmay include dielectric polymeric-based materials, such as polyethyleneterephthalate (PET, PETP), polyimides, or abrasion-resistantpolysiloxanes. Alternatively inorganic materials, such as glass orceramics may be used. It is appreciated that a photoconductive materialcould be used for outer dielectric imaging layer 824 in accordance withcertain embodiments of the invention.

The conductive backing 826 preferably comprises densely spaced thinconductive electrodes. The density of the electrodes, the transversecross-sectional geometry thereof and the thickness of the outerdielectric imaging layer 824 determine the spatial resolution of thelatent image in the axial direction parallel to the longitudinal axis830 of the drum 820.

Contained within drum 820 are imaging electronics which preferablyincludes serial to parallel data conversion electronics and high voltageelectrode drivers.

Alternatively or additionally, drum 820 may contain imaging electronicswhich include parallel to serial data conversion and transmissionelectronics; and sample and hold circuitry for sensing signalinformation from outer dielectric imaging layer 824. It is appreciatedthat this type of imaging electronics is useful for image reading asdescribed hereinbelow.

External data representing the latent image to be generated may besupplied to the imaging electronics through one or several connectors832. It is appreciated that connectors 832 may be located additionallyor alternatively on the faces of the drum or on a drum axle 834.Alternatively or additionally, data connections between the rotatingdrum and the stationary data source may be effected using slip rings(not shown), which may include mechanical, optical or conductive liquidbased elements. Alternatively in accordance with certain configurations,where the drum does not complete full rotations, information may betransferred through a flexible cable.

The length of the drum along axis 830 may be any desired length and istypically selected to conform to standard print formats, for example,standard A and B formats.

The printing properties of the drum, its durability, the speed ofoperation, the toning materials, the operating voltages and the printcycle are determined in part by the materials used in building the drum.For example, drums with an outer dielectric glass or ceramic imagingsurface are expected to provide enhanced durability.

In accordance with an alternate image reading embodiment of the presentinvention, outer dielectric imaging layer 824 can be replaced by anouter photoconductive layer (not shown). In this embodiment, imagingelectronics for image reading applications, as described hereinabove andhereinbelow, are required.

Reference is now made to FIGS. 15A-15D which illustrate four alternativearrangements of imaging regions on the cylindrical surface of drum 820.

As noted above, the surface of drum 820 comprises one or more imagingregions with the remaining surface of the drum not being active in theimaging process. The size of the non-active surface area is selected toachieve the desired drum diameter in accordance with a desired printcycle and print engine configuration. The division between imaging andnon-imaging regions thus depends on the printing cycle, printing speedand other characteristics of a given printing engine. Possibleconfigurations include, for example:

1. A single imaging region extends across less than half of the drumsurface. One printing cycle is carried out per complete drum rotation.In this case, the drum typically continuously rotates in a singledirection. This embodiment is shown in FIG. 15A where the conductivebacking is indicated by reference numeral 836.

2. Two distinct independent imaging regions are separated by non-imagingregions. Two printing cycles are carried out per complete drum rotation,one cycle for each imaging region. In this case, the drum typicallycontinuously rotates in a single direction. This embodiment is shown inFIG. 15B where the conductive backing portions are indicated byreference numerals 838 and 840.

3. A single imaging region extends across most or all of the drumsurface. Each printing cycle is accomplished by several rotations of thedrum. In this case, the drum typically continuously rotates in a singledirection. Alternatively, a printing cycle may be carried out by acombination of clockwise and counterclockwise rotations of the drum.These embodiments are shown in FIGS. 15C and 15D, where the conductivebacking portions are indicated by reference numerals 842 and 844.

Typically, the imaging regions and non-imaging regions have similarexternal appearance. Alternatively, the regions may be externallydistinguishable.

Reference is now made to FIGS. 16A-16E which illustrate preferredinterior structures for drum 820. Referring first to FIG. 16A, it isseen that the drum, indicated by reference numeral 846, comprises aconductive backing 847 comprising closely spaced electrodes 848 embeddedin a dielectric substrate 850, imaging electronics located on primaryprinted circuit boards 852 located interior of the drum and connectors854 connecting the electrodes 848 to the printed circuit boards 852. Theelectrodes 848 and connectors 854 are shown in FIG. 16A with exaggerateddimensions and spacing. In reality, each electrode has a width ofapproximately 20-40 microns and-the electrodes are separated from oneanother by a spacing of approximately 10-20 microns. The connectors 854may be pads formed by printed circuit techniques. In reality, the widthof each connector 854 is typically on the order of 100 microns and thegap between connectors is also typically on the order of 100 microns.

The dielectric substrate 850 having electrodes 848 embedded therein maybe fabricated by any suitable technique including, for example,techniques mentioned hereinabove.

Two general methods for producing the conductive backing 847 of theimaging region of the drum are described hereinbelow with reference toFIGS. 16C and 16D.

Drum 846 comprises an imaging region 856 which extends across most ofthe drum surface and a non-imaging region 858. Non-imaging region 858covers the connectors 854 which connect conductive electrodes 848 toprimary printed circuit boards 852. Each primary printed circuit board852 typically contains a single line array of connectors 854.

Typically, primary printed circuit boards 852 are shaped to fit insidedrum 846. The line array of connectors 854 of each printed circuit board852 is positioned for exposure through a narrow slot 860 in the drumsurface, which slot extends parallel to the longitudinal axis of drum846. The number of slots 860 may correspond to the number of primaryprinted circuit boards 852. Alternately, each slot 860 may correspond tomore than one primary printed circuit board 852.

Imaging electronics, including digital serial-to-parallel data transferand high-voltage electrode drivers, may be located on a non-connectorsection of printed circuit board 852. The non-connector section ofprimary printed circuit boards 852 may be flexible or rigid depending onthe drum structure and diameter.

Reference is now made particularly to FIG. 16B which shows analternative inner configuration of drum 846. In this configuration, twosets of parallel printed circuit boards are used, a primary set 862 anda secondary set 864. Each printed circuit board in primary set 862 isconnected to a plurality of conductive electrodes 848 (FIG. 16A) bymeans of a single line array of connectors. Preferably, the primary setof printed circuit boards 862 does not contain any electroniccomponents. Instead, the primary set of printed circuit boards 862serves only as an interconnection between conductive electrodes 848 andthe secondary set of printed circuit boards 864.

Alternatively, a pre-formed rigid base, comprising any suitabledielectric material (for example ceramic, glass, anodized aluminum,etc.) with a surface pattern created using a conductive material (forexample copper, gold, etc.) may be used in place of the primary set ofprinted circuit boards 862.

The secondary set of printed circuit boards 864, which may be made of aflexible or rigid material, comprises imaging circuitry and a set ofconnectors. The number and layout of connectors on the secondary set ofprinted circuit boards 864 corresponds to that on the primary set 862.

Electrical connection in registration between the connectors of theprinted circuit boards of the primary set 862 and of the secondary set864 may be effected using elastomeric high density contact arrays (zebraconnectors) 866. Alternately, any other suitable method for generatinghigh density electrical connections between the two sets of printedcircuit boards may be used.

It is appreciated that connectors may be placed on one or both sides ofthe printed circuit boards of the primary set 862. Typically, contactbetween the set of connectors on the primary and secondary sets may becarried out through application of pressure.

It is appreciated that the number of printed circuit boards 864comprising the secondary set may be determined in accordance with thespecific mechanical structural considerations of drum 846 and designconsiderations of the printed circuit boards. For example, numeroussecondary printed circuit boards 864 could be arranged in a cascade andconnected to one printed circuit board of the primary set 862 allowingfor simplification of each secondary printed circuit board 864.

It is further appreciated that in accordance with this configuration,printed circuit boards of the secondary set 864 are removable from drum846 by releasing the pressure holding the connectors. Thus, the outerpart of drum 846 may be replaced without necessitating replacement ofthe secondary set 864 of printed circuit boards or of the imagingelectronics formed thereon.

Reference is now made to FIG. 16C which illustrates conductiveelectrodes 848 of drum 846 at an intermediate stage of fabrication.

A conductive wire 868 is tightly wrapped around an inner dielectricsurface 850 of drum 846 resulting in a coiling of the wire intospacewound turns about surface 850 with a very precise pitch. Prior towrapping the wires, a thin adhesive layer (not shown) may be adhered tothe inner dielectric surface 870 of the drum 846 to ensure properalignment and spacing of the wires.

The wires should exhibit sufficiently high electrical conductivitycombined with mechanical strength so as not to tear during the windingprocess. The wires may be made of stainless steel, copper alloys,tungsten, etc. and may be uncoated. Alternatively, the wires may becoated with an insulating material such as glass or alternatively with apolymeric coating such as polyurethane, polyimide, etc. The actualmaterial for the wire and for its coating are selected to best suit thefabrication process, the desired properties of the printing device andother materials used in production.

The pitch of the coil is selected to match the spacing of connectors 872of the first primary printed circuit board 876 which is located in aslot 877. The wire is coiled until each connector 872 is located incontact with an individual turn of the wire. Primary printed circuitboards 878 may be of the type described above in conjunction with FIG.16A or FIG. 16B.

After the wire is coiled so that each connector 872 of the first primaryprinted circuit board 876 is located in contact with an individual coilof the wire, the wire is bonded to connectors 872. Bonding may beaccomplished by parallel gap bonding techniques. Alternatively, bondingmay be carried out by employing reflow wire soldering techniques usinghot rams, or non-contact hot air jets. Where necessary, the insulatingcoating may be stripped off the segments of the wire that are located inthe connector region, during or after the winding process. For wireswith a solderable coating, stripping may be carried out during thesoldering process. Additionally, or alternatively any other suitablebonding technique may be used.

In accordance with a further embodiment of the invention, connectors 872of the primary printed circuit boards 876 may be precoated with apredetermined amount of soldering material per connector, for example,thin-lead solder. This method facilitates the bonding process forfine-pitch soldering.

After the wire has been bonded to all the corresponding connectors in asingle line array 874, a thin layer of dielectric material encapsulatesthe line array 874 of connectors 872 to electrically insulate them fromeach other.

The wire coil is then cut parallel to the drum axis adjacent both sidesof an adjacent slot 879. The cuts disconnect from each other adjacentportions of the coil in electrical contact with adjacent connectors of agiven line array 874. The result is an array of precisely spacedmutually electrically-insulated conductive wires each of which isseparately connected to a separate connector forming part of array 874.These wires constitute some of the electrodes of the present invention.Additionally, the cuts remove those portions of wire which wouldotherwise extend over an adjacent line array 880 of connectors 882 of anadjacent primary printed circuit board 884.

After the above steps have been completed on the first primary printedcircuit board 876, they are repeated for every primary printed circuitboard 878 typically continuing until each connector on each primaryprinted circuit board is in electrical contact with a wire. The spacingbetween connectors in each single line array on each primary printedcircuit board is constant. However, the first connector and thus allsubsequent connectors in the line array on each primary circuit boardare staggered along an axis parallel to the longitudinal axis of thedrum slightly relative to the connectors of each adjacent primaryprinted circuit board.

The number of connectors used, which determines the pitch of thewinding, can be varied to achieve different spatial resolutions alongthe drum axis direction. Using several primary printed circuit boardsand several sequential corresponding winding steps enables high spatialresolution of the conductive electrodes to be realized while enablingrelatively lower resolution to be employed for the connectors, whichresolutions are better suited to printed circuit board manufacturing andbonding processes. Alternatively, when lower resolutions are required,or when otherwise desirable, a single primary printed circuit board canbe used.

Densities greater than 600 lines (wires) per linear inch can be achievedusing this technique, as can be appreciated by a consideration of theexamples shown in FIGS. 16A, 16B and 16C, where four primary printedcircuit boards and 1600 connectors are employed in association with eachprimary printed circuit board.

Alternatively, the conductive backing 847 of drum 846 may be prepared byother techniques which allow the creation of densely packed conductiveelectrodes.

Reference is now made particularly to FIGS. 16D and 16E, whichillustrate an alternative configuration of the conductive backing andthe inner structure of drum 820 in accordance with an embodiment of thepresent invention.

The conductive backing comprises a multi-layer blanket 886 whichincludes a flexible dielectric carrier 890 and a further layer 894 onwhich is formed a plurality of conductive electrodes 896, where theelectrodes terminate in a fan out array 898 of connectors. Conductiveelectrodes 896 together with fan out array 898 may be produced on layer894 by photoetching, plasma etching, laser etching, mechanical etching,electroforming or a combination thereof.

Blanket 886 is wrapped around inner layer 828 of drum 820, with eachblanket end being inserted into a opening 902. Fan out arrays 898 at theblanket ends are aligned with a secondary set of printed circuit boards904 that are located within drum 820. Fan out arrays 898 at each blanketend may comprise one or more contact regions 906. Through the contactregions 906, electrodes of blanket 886 are electrically connected tosecondary printed circuit boards 904. Typically, the electricalconnection may be made using elastomeric contact arrays as describedhereinabove. Alternately any other high density connectors may be used.Blanket 886 eliminates the need for a set of primary printed circuitboards. A thin adhesive layer may be used to ensure adhesion ofdielectric carrier 890 to drum inner layer 828.

After a conductive backing has been produced by any of the methodsdescribed hereinabove with reference to any of FIGS. 16A-16E, or by anysuitable alternate method, outer dielectric surface 824 is formed. Outerdielectric surface 824 is created by coating conductive backing 826 witha dielectric layer or layers resulting in a uniform smooth outer layerwith predetermined total thickness.

The capacitance of outer dielectric surface 824 is determined by thedielectric constant of the material or materials used in the coatingprocess and the accumulated thickness of all of the coating layers.

Typically, the total thickness of outer dielectric surface 824 isbetween 10-50 microns. The capacitance of outer dielectric surface 824plays an important role in determining the final resolution of thelatent image and the maximum charge density that can be accumulated onthe surface per the voltage applied to the conductive backing.

In addition to its dielectric properties, the material used for theouter dielectric coating should have appropriate surface energy, besufficiently durable and abrasive resistant.

Various techniques may be used to create outer dielectric imaging layer824 of drum 820. The specific method used is chosen to conform to theconfiguration of conductive backing 826 and the specific materials used.Prior to coating, the surface of conductive backing 826 may bechemically treated to prepare for coating with one or many thindielectric layers.

Coating techniques may comprise fog spraying of dielectricpolymeric-based solutions or dispersions. Typically, fog spraying iscarried out while the drum is rotating. Alternatively, coating may becarried out using dip or roll coating techniques. Depending on thematerial of the conductive electrodes, either dip or roll coating (witha polymeric based solution, dispersion or two components) or hot meltdip or roll coating (using dielectric thermoplastic materials or glass)may be used.

After coating, each dielectric layer may be hardened using evaporation,a thermal process, or curing via radiation or heat, depending on thecoating material used.

After all dielectric layers have been coated and hardened, a smoothuniform outer dielectric imaging layer 824, having an embeddedconductive backing 826, remains.

Alternatively, outer dielectric surface 824 may be created by a roughcasting of polymeric based materials or sintering of ceramics inside apreformed container. After the casting or sintering is complete, thecontainer is removed, and all unnecessary coating material is machinedaway leaving a coating with a specified even thickness and a smoothouter layer.

Furthermore, outer dielectric surface 824 may be created by depositiontechniques, including vacuum or plasma deposition to deposit a suitabledielectric material. Typically, deposition is carried out while the drumis rotating.

Alternatvely, coating techniques may comprise spreading of a dielectricadhesive onto the electrodes of conductive backing 826, thus filling allgaps. A thin dielectric film (such as polyester based films) ofpredetermined thickness may then be wrapped around the entire drumsurface and pressure laminated to the conductive wires/adhesive surface.Following this step, the adhesive is cured leaving a flat dielectricouter imaging surface adhered to the electrodes.

Optionally any and all of the drums described herein may be providedwith an additional thin dielectric outer coating, which preferably isdisposable and easily replaceable. This coating, which may be formed ofpolyethylene terephthalate (PET, PETP) or any other suitable material,may have a thickness in the general range of 10 microns.

In accordance with an alternate embodiment of the present invention, forimage reading applications, an outer photoconductive layer (not shown)may be formed over the electrodes of conductive backing 826 by anysuitable techniques.

Reference is now made to FIG. 17A which is a simplified schematicillustration of the imaging electronics of the printed circuit boards.The imaging electronics comprise a cascade 909 of multi-channel serialto parallel devices outputting to drivers 910. Each driver 910 providesa high voltage output in the range of up to several hundred volts, whichdrives a separate conductive electrode 912.

Data is input serially from an external data source, such as a computer,copier, scanner or facsimile receiver, to the imaging electronics via adata bus 913. Serial data, representing a pattern which is to begenerated on the outer dielectric imaging surface of the drum, istypically fed to data bus 913 in 1, 8 or 10 byte words.

The data propagates along the cascade 909 of mult-channel serial toparallel converting devices. A single output of each device withincascade 909 corresponds to each conductive electrode 912. For example,if the conductive backing of a drum contains 6400 conductive electrodes,one hundred 64-channel devices or two hundred 32-channel devices may beemployed in the cascade 909. Typically, the devices are evenlydistributed over the printed circuit boards on which the imagingelectronics are located.

After data representing one raster line of a pattern to be generated hasbeen serial loaded across the cascade of mult-channel serial to parallelconverters 909, the data is loaded in parallel to high voltage drivers910. Based on the data, high voltage drivers 910 apply appropriatevoltages to the electrodes.

Simultaneous with the application of the voltages to conductiveelectrodes 912, an Elongate Alternating Polarity Charge Source (EAPCS),of the type described in any of FIGS. 7A-11C, is activated. It isappreciated that when an EAPCS of the type described in FIG. 11C isused, continuous operation as described herein is used.

The EAPCS may be placed in proximity to the outer dielectric imagingsurface of a drum, such as drum 820 of FIG. 14A or drum 846 of FIG. 16A,and includes at least one well-defined edge. The EAPCS is positioned sothat, during drum rotations, the trailing edge of the EAPCS is awell-defined edge. Charge is not supplied by the EAPCS to areas of theouter dielectric imaging surface of the drum that are beyond the edge ofthe EAPCS.

The EAPCS may be activated for a pulse duration containing tens ofplasma cycles. This activation supplies charges to the outer dielectricimaging surface. The final charge retained on the outer dielectricimaging surface of the drum is a dynamic function which corresponds tothe voltage signals applied to conductive electrodes 912 over the pulseduration. The area of charge retention is bounded by the defined edge ofthe EAPCS.

During EAPCS activation, data representing the next raster line of theimage to be generated is serially loaded into cascade 909, but the datais not forwarded to high voltage drivers 910.

Prior to application of voltages corresponding to data for a subsequentraster line of the image, the drum rotates slightly to position theEAPCS for the next line. After rotation of the drum, the new line datais sent to high voltage drivers 910 and the EAPCS is activated, againcausing retention of a new amount of charge, positive or negative,corresponding to the new line data that was fed to the devices. Areasthat are beyond the generally linear boundary created by the definededge of the charge source do not receive additional charge. Instead,they retain the charge that was previously accumulated.

The above-described line writing cycle repeats itself until a latentimage of the entire pattern is generated.

Alternafively, the EAPCS may be operated continuously during the latentimage generation process. During continuous operation of the EAPCS, theapparent surface voltage (ASV) of the outer dielectric surface iscontinuously refreshed, thus the retained charge pattern is continuouslyrefreshed. Conversely, when the EAPCS is operated in pulses, the ASV issupplied in bursts, with the retained charge pattern being refreshedduring time intervals whose length determines the dimension of a singleraster line of the image in the direction in which the drum rotates.

Reference is now made to FIG. 17B which is a simplified schematicillustration of imaging electronics of the printed circuit boards foruse in reading electrostatic charge images. The imaging electronicscomprise sample and hold circuitry 915 which supply output to a cascade917 of multi-channel parallel to serial data conversion and transmissiondevices. Each sample and hold circuit 915 receives input from aconductive electrode 912.

A data bus 918 serially outputs data to an external data collector. Eachconductive electrode 912 is electrically connected to a sample and holdcircuit 915, which are typically evenly distributed over the printedcircuit boards on which the imaging electronics are located.

The data output by data bus 918 represents digital informationcorresponding to an electrostatic charge image which was read from theouter dielectric imaging surface of the drum in accordance withtechniques described herein.

It is appreciated that the image to be read may be written onto thedielectric outer imaging surface using the dynamic charge retentiontechniques described herein or any other suitable technique forgeneration of latent charge images. For example, when the outerdielectric imaging surface is a photoconductor as described hereinabove,a charged latent image can be created on the surface using conventionalelectrophotography techniques.

An additional benefit to the image reading techniques described hereinis that the reading process effectively erases and uniformly charges theouter dielectric imaging surface in a single step, pre-conditioning theouter dielectric imaging surface. Such pre-conditioning is not necessarywhen writing images using the dynamic charge retention techniquesdescribed herein, but is required prior to the projection of opticalimages on a photoconductor.

An Elongate Alternating Polarity Charge Source (EAPCS), of the typedescribed in any of FIGS. 7A-11C, is placed adjacent an outer dielectricimaging surface of a drum, such as drum 820 of FIG. 14A or drum 846 ofFIG. 16A. It is appreciated that during drum rotations for imagereading, the leading edge of the EAPCS is a well-defined edge. The EAPCSdoes not supply charge to areas of the outer dielectric imaging surfaceof the drum that are beyond the leading edge.

To read a single line of information from outer dielectric imagingsurface, the EAPCS may be activated for a pulse duration containing tensof plasma cycles. This activation unifies the charge level across oneline of the electrostatic charge pattern, erasing that line of thepattern, and inducing a current flow from each conductive electrode 912to a capacitor (not shown) in the corresponding sample and hold circuits915. During each line reading cycle, the total charge flow into eachsample and hold circuit 915 is a function of the charge density levelthat was present at the location on the outer dielectric surfacecorresponding to the conductive electrode 912 connected to the sampleand hold circuit 915, prior to activation of the EAPCS.

The charge level retained on the line of the outer dielectric imagingsurface after line reading is a function of the bias voltage applied tothe conductive electrodes during line reading. Typically conductiveelectrodes 912 are grounded during line reading. Alternately, conductiveelectrodes 912 may be biased to a predetermined bias voltage level. Itis appreciated that the same level of voltage is typically applied toall conductive electrodes 912 during reading.

Signals from all the sample and hold circuits 915 are output in parallelto a cascade 917 of multi-channel parallel to serial data conversion andtransmission devices. Cascade 917 provides digital output datarepresenting one line of the image that was present on outer dielectricimaging surface. Data on output bus 918 typically comprises one or eightbits of information, representing 2 or 256 grey levels, respectively.

The above-described line reading cycle repeats itself until digitalinformation representing the entire electrostatic charge image to beread is collected.

During pulse operation of the EAPCS, the pulse duration is synchronizedwith the sample and hold circuit cycle. The duration of the pulsedefines the dimension of the raster line of the image that is beingread.

Alternatively, the EAPCS may be operated continuously during the imagereading process. During continuous operation of the EAPCS, the dimensionof a single raster line of the image being read is determined by thesample and hold circuit cycle.

It is appreciated that the abovedescribed image reading techniques mayprovide numerous functions including optical scanning and reading, andelectrostatic information storage.

Reference is now made to FIG. 18 which illustrates an image patterncomprising a plurality of pixels 919 along conductive electrodes 912,where the charge density of each pixel is determined by input data.

It is appreciated that the imaging electronics may be configured fordifferent methods of achieving grey shades for printing images. Specificprinting methods may include optical density modulation or pixel sizemodulation and micropositioning as described hereinabove, all of whichare compatible with the drum configuration and imaging techniquesdescribed hereinabove. It is appreciated that a large number of greyshades can be achieved without resolution sacrifices thereby allowingprint outs which combine high quality text, graphics and images.

Optical density modulation allows images to be created by controllingthe amount of toner, hence the shade, of each pixel. Alternately, imagesmay be created using super-pixel half-tone patterns, where eachsuper-pixel comprises several pixels and where each pixel is eithertoned or non-toned. In addition, micropositioning of each pixel may becombined with super pixel half tone pattern techniques.

It is further appreciated that in accordance with the writing techniquesdescribed herein, the imaging electronics and the level of high voltageemployed can accommodate a variety of toners and toning techniques,including liquid and dry toners.

One method for achieving continuous tone images using drum 820 and thewriting techniques described hereinabove is described hereinbelow.

Through amplitude modulation of the voltages applied to the electrodes912 by the drivers 910 during EAPCS activation, the total amount ofcharge retained may be precisely determined.

A charge pattern containing a two-dimensional array of pixels, where thecharge density for each pixel can be selected, is created using thelatent image writing techniques and drum described hereinabove. Eachpixel is defined as the minimum unit that can be addressed in a line.The number of pixels in the direction of the longitudinal axis of thedrum is equal to the number of conductive electrodes therein. The shapeof the pixels is determined by the geometry of the conductiveelectrodes, the thickness of the outer dielectric layer, and the edgedefinition of the EAPCS. Using appropriate toners and toning techniques,a continuum of shades can be achieved for each pixel.

A benefit of creating continuous tone images using the writingtechniques and drum described hereinabove is that the total chargedensity at any pixel location is a function of the voltage applied tothe conductive electrode during EAPCS activation. Therefore, the chargedensity is not sensitive to environmental factors, such as temperature,humidity, light, etc. Typically, line-printing techniques, for exampleLED Arrays in electrophotography, ion guns in ionography, are based onimagewise writing heads which are typically composed of pixel- sizedwriting sources. Dissimilarities between any of the pixel- sized writingsources comprising an imaging head may result in image non-uniformities.

The line printing techniques described herein do not employ an imagewisewriting head. Instead, the charge density for each pixel location isdetermined by the voltage applied to the conductive electrodes. Ordinaryfluctuations in the charge source do not cause non-uniformities in theimage. Therefore, the charge density can be closely controlled andrepeatability is achievable.

Reference is now made to FIG. 18A which illustrates an example of thecircuitry of basic continuous tone imaging apparatus that can be used toperform amplitude modulation for each conductive electrode 912. FIG. 19Adepicts one preferred embodiment of the apparatus of FIG. 17A, which isparticularly suitable for use in generating continuous tone images.

The basic circuitry typically comprises one among a multiplicity ofunits which make up the cascade 917 (FIG. 17A). Typically, for highquality continuous tone imaging, 8 bit inputs to the data bus are used,where each 8-bit data word corresponds to the desired voltage level forone conductive electrode. When 8-bit word input is used, 256 differentvoltage levels are available for each conductive electrode correspondingto 256 different shades for each pixel. Since up to four differentlatent images, one representing each different print color, such ascyan, magenta, yellow, black, may be used for each final color printimage, 256 shades for each color translates to millions of possiblecolor combinations.

Data is serially loaded from data bus 913 and propagated at a high rate,typically using loading clock 922, which operates at few tens of MHz,into each data latch 924 of each device. Each data latch 924 correspondsto one secondary latch 926 and one output voltage stage 928.

Once an entire line of data has been loaded into the data latches 924,the line data is transferred in parallel from data latches 924 tosecondary latches 926. Transferring the data to secondary latches 926allows new data representing the next line of the image to be loadedinto the data latches 924. Concurrently, all data in secondary latches926 undergoes a parallel digital to analog high voltage conversion.

The outputs of secondary latches 926 are supplied to respective gatebuffers 927. The outputs of gate buffers 927 are supplied to respectiveoutput stages 928. A control unit 930 controls the timing and triggeringof the subsystems in each device.

The conversion cycle begins by setting a binary reference counter 934 oneach device to 00000000. In predefined increments, a clock signal issent from control unit 930 to reference counter 934 incrementingreference counter 934 by units of 1 until a maximum level of 11111111 isreached. Each increment is associated with one of 256 possible outputlevels.

Reference is now made to FIG. 19B which is a detailed circuit diagram ofa possible output stage 928 of the basic continuous tone circuitryapparatus of FIG. 19A. It is appreciated that other circuits whichperform a multi-channel digital to high voltage analog conversion mayalso be used.

At output stage 928, which employs high-voltage CMOS technology, digitaldata from secondary latch 926 is converted to an analog output voltageV_(out) as follows:

A ramping voltage V_(ref) typically of up to 600 Volts, is applied tooutput stages 928 of all devices in cascade 917. Voltage V_(ref) isswitched via charging transistors 936 causing hold capacitors 938 ineach output stage 928 to experience a rise in voltage.

Each time the value in reference counter 934 is incremented, the digitalinformation in secondary latch 926 associated with each output stage 928is compared with the value in reference counter 934. When referencecounter 934 reaches the value held by secondary latch 926, the chargingtransistor 936 is switched off thereby terminating charging of the holdcapacitor 938 of that output stage 928.

When charging transistors 936 are switched off, V_(ref) may still beramping. Voltage V_(g) at the gate of voltage follower 940 begins toramp and continues ramping at the same rate as voltage V_(ref). Sincevoltage follower 940 acts as a follower of V_(g), the output voltageVout also begins to ramp at the same rate as V_(g) and V_(ref). WhenV_(ref) reaches its peak and begins holding, V_(g) also reaches its peakand begins holding. V_(out) follows V_(g) and reaches its peak at thesame time. The overall effect of this operation is a conversion of thedigital data to an analog high voltage output value V_(out).

An advantage to the stage described hereinabove is that there is nocross-coupling between different channels, since all of the outputsreach their peak voltages at the same time.

Typically, V_(out) is maintained constant across conductive electrodes912 during the EAPCS activation. Once the EAPCS is made non-active,V_(ref) is reduced to zero, allowing hold capacitors 938 to discharge,thereby discharging the corresponding conductive electrode 912. Thetotal charge accumulated on a given region of the conductive electrode912 prior to discharge is determined by the data input to data latch 924associated with the corresponding output stage 928. After V_(ref) risesback to its normal level, a conversion cycle for the next line of datais begun.

Reference is now made to FIG. 20 which illustrates a half-tone imagepattern comprising a plurality of pixels 950, delimited by dashed lines,along conductive electrodes 912 (FIG. 17A). The image pattern may begenerated using pulsewidth modulation techniques and image writingtechniques described hereinabove where an EAPCS is activatedcontinuously during the image writing cycle.

Pulsewidth modulation includes controlling the fractional area to becharged in each pixel 950 and micropositioning of the charged areaswithin pixel 950. In the example shown, the charged areas of the pixelare shaded and the substantially uncharged areas appear unshaded.Typically, the charge density in charged areas is the saturation levelcorresponding to the voltage applied to conductive electrodes 912.

Reference is now made to FIGS. 21A and 21B which illustrate an exampleof basic pulsewidth modulation imaging apparatus that can be used toperform pulsewidth modulation for imaging pixels generated on conductiveelectrodes 912 (FIG. 17A). FIGS. 21A and 21B depict a further preferredembodiment of the apparatus of FIG. 17A which is particularly suitablefor use in generating half tone images having multiple gray levels.

The basic circuitry typically comprises one among a multiplicity ofunits which make up the cascade 917 (FIG. 17A). Typically, for half-toneimages with multiple gray levels, 10 bit inputs to the data bus may beused. Eight of the ten bits represent 256 possible levels for the fillfraction of each pixel. The remaining two bits represent fourpossibilities for micropositioning of the fill area within the pixel.Altematively, any other suitable multi-bit input data combination may beused, with certain bits representing the fill fraction and the remainingbits representing the positioning.

Micropositioning of the fill area is used to achieve images having highspatial frequency. High spatial frequency enhances the appearance oftoned images and enables simulation of continuous tone in half-toneimages. Four possibilities for micropositioning represents a reasonableminimum necessary to ensure desirable variation in the fill location atadjacent pixels located on adjacent conductive strips as described inFIG. 6A-6C above. For a fill area that is greater than half the pixelarea, the location may be adjacent the leading or trailing edge of thepixel. For a fill area that is less than half the pixel area, thelocation typically begins or ends at the middle of the pixel and extendstowards the leading or trailing edge.

Conversion of input data to voltages on the conductive strips 912 bymeans of pulse width modulation is preferably carried out as describedhereinbelow:

Data is serially loaded from a data bus 952 and propagated at a highrate, typically using loading clock 954 which operates at a few tens ofMHz, into each data latch 956 of each unit in cascade 917. Each datalatch 956 corresponds to one secondary latch 958 comprising a converterand comparator and one output stage 960. Once an entire line of data hasbeen loaded into data latches 956, the line data is transferred inparallel from data latch 956 to secondary latches 958.

Transferring the data to secondary latches 958 allows new datarepresenting the next line of the image to be loaded into data latches956. Based on the above-described positioning bits, the data insecondary latches 958 is typically converted to two words describing thebeginning and end locations of the fill region of the pixel.

The outputs of secondary latches 958 are supplied to respective gatebuffers 966. The outputs of gate buffers 966 are supplied to respectiveoutput stages 960. A control unit 964 controls the timing and triggeringof the subsystems in each device.

The conversion cycle begins by setting a binary reference counter 962 oneach device to 00000000. In predefined increments, a clock signal issent from control unit 964 to reference counter 962, incrementingreference counter 962 by units of 1 until a maximum level of 11111111 isreached. Each increment is associated with one of 256 possible pulsewidth outputs.

Each time the value in reference counter 962 is incremented, the twoword digital information in secondary latch 958 associated with eachoutput stage 960 and the value in reference counter 934 are compared.When reference counter 962 reaches the value held by secondary latch 958for the beginning location of the pixel fill area, a signal is sent fromsecondary latch 958 to output gate buffer 966 and the output stagevoltage increases to V_(HV). When reference counter 962 reaches thevalue held by secondary latch 958 for the end location of the pixel fillarea, a signal is sent from secondary latch 958 to output gate buffer966 and the output stage voltage goes to ground.

Typically output stage 960 comprises push-pull high voltage MOSFETScapable of sinking and sourcing current from conductive electrodes 912to which the output stages 960 are connected.

It is appreciated that half-tone color images can be generated bycreating one half-tone image for each of the four printing colors.

It is further appreciated that during half-tone printing using thepulsewidth modulation imaging techniques described hereinabove,relatively large numbers of grey levels are achieved without reducingspatial resolution. Typically, half-tone techniques using super-pixeland screening methods require a tradeoff between the number of graylevels and the spatial resolution.

Reference is now made to FIG. 22 which illustrates an example ofcircuitry of basic apparatus that can be used to generate an alternatetype of half-tone image on conductive electrodes 912. FIG. 22 depicts afurther preferred embodiment of the apparatus of FIG. 17A which isparticularly suitable for use in generating super-pixel half tone imagesbased on screening methods.

The basic circuitry typically comprises one among a multiplicity ofunits which make up the cascade 917 (FIG. 17A). Typically, each suchunit receives a one-bit input from a data bus 970. Based on the binaryinformation supplied by the data bit, each conductive electrode 912receives either high voltage or low voltage during the EAPCS activation.In this embodiment, the EAPCS can be activated in pulse mode orcontinuous mode.

Data is serially loaded from a data bus 970 and propagated at a highrate, typically using a loading clock 972 which operates at few tens ofMHz, into a shift register 974. Once all the data for one line has beenloaded into shift register 974 of all units in the cascade 917, the datais transferred in parallel to secondary latches 976. A signal is sentfrom a secondary latch 976 to an output gate buffer 978 indicatingwhether output stage 980 should apply high or low voltage to conductivestrip 912.

Typically output stage 980 comprises push-pull high voltage MOSFETScapable of sinking and sourcing current from conductive electrodes 912to which the output stages 980 are connected. Output stage 980 may be ofthe type shown in FIG. 21B.

Reference is now made to FIG. 23 which is a schematic illustration ofprinting engine apparatus which may be incorporated into a variety ofprinter and digital copy systems. The printing engine apparatus, whichmay be used for tandem or satellite systems, is based on use of adigital input electrostatic (D/E) imaging drum as described hereinaboveand is characterized by its ability to provide high quality text,graphics, and images using continuous tone or half tone imagingtechniques. These imaging techniques may include modulation of theoptical density of each pixel or modulation of the pixel dimensions asdescribed hereinabove or any other suitable technique that is capable ofproviding a large number of grey levels.

The printing engine apparatus comprises an imaging drum 1010, having adielectric outer surface 1011, and a latent image generation unit 1012which creates electrostatic latent images on the dielectric outersurface 1011 of imaging drum 1010 in accordance with the techniques andlatent image generation methods and using the apparatus describedhereinabove and/or in one or more of applicant's co-pending patents andpatent applications including U.S. Pat. Nos. 5,289,214 and 5,157,423 andU.S. Pat. applications 12939, 12466), the disclosure of which is herebyincorporated by reference.

Typically, the printing engine apparatus further comprises subsystemsfor image developing, image transfer, image fixing and cleaning ofresidual toner from imaging drum 1010. A step of electrostaticpreconditioning of the dielectric outer surface of a drum prior towriting of a latent charge image, typically associated with standardelectrostatic imaging techniques, is eliminated when using the latentimage generating techniques described herein.

In FIG. 23, each subsystem is depicted as an individual unit with adeveloping unit being indicated by reference numeral 1016, a transferunit being indicated by reference numeral 1018, a fixing unit beingindicated by reference numeral 1020 and a cleaning unit being indicatedby reference numeral 1022. Developing, transfer, fixing and cleaningtechniques may be selected in accordance with the printer or digitalcopy system being used and in accordance with the desired printingmedium, print quality, and process speed.

Developing unit 1016 is operative to develop an electrostatic latentimage present on dielectric outer surface 1011 of drum 1010 to a visibletoned image by application of liquid or dry toner. Developing unit 1016may comprise one or several developing elements where each developingelement applies a unique toning color.

The printing engine apparatus may be adapted for use with the differingdeveloping voltage requirements of a multitude of toners. This is due tothe fact that the image generating techniques which are used to createlatent images on the dielectric outer surface 1011 of imaging drum 1010can be adapted to a variety of apparent surface voltage ranges, asdescribed hereinabove.

Either dry powder toner or liquid toner may be applied by developingunit 1016, with the precise structure of developing unit 1016 beingselected in accordance with the type of toner used and the specificprinter requirements.

Types of dry powder toners which may be used in the developing unit 1016include monocomponent dry powder toners, of assorted size distributions.Specific types of toner may include both conductive and dielectricmonocomponent toners such as those described in "Trends in ImagingMaterials for Color Hardcopy", by D. Wilson, SPIE Vol. 1253 Hard Copyand Printing Materials, Media and Process (1990). Generally, when usingdry powder toner, contact between developing unit 1016 and dielectricouter surface 1011 is required.

In place of monocomponent dry powder toners, dual component dry powdertoners such as those described in the aforesaid SPIE article may beused. When dual component dry powder toners are used, the developingunit 1016 may operate using non-contact developing techniques such asdescribed, for example in U.S. Pat. No. 4,746,589 entitled DevelopingMethod in Electrophotography Using Oscillating Electric Field to Hanedaet al.

According to a further alternate embodiment, electrostatic liquid tonersmay be used in the developing unit 1016. Liquid toners having varioustypes of colloidal dispersions with various size distributions,rheologic and fixing properties may be used. Primary fixing propertiesof the liquid toners may include drying by evaporation, heat andpressure fixing.

Transfer unit 1018 is operative to transfer the visible toned image fromthe dielectric outer surface 1011 of imaging drum 1010 to a print medium(not shown). A variety of techniques may be used to transfer the tonedimage from imaging drum 1010 to the print medium. Examples of suchtechniques are briefly described hereinbelow:

A monochrome image (typically cyan, yellow, magenta, or black) isdeveloped on the dielectric outer surface 1011 of imaging drum 1010 inaccordance with the steps described hereinabove. After developing, themonochrome image is transferred from the dielectric outer surface 1011of imaging drum 1010 directly to print medium. The transfer may becarried out electrostatically, mechanically, or through a combinationthereof.

To generate color images, several monochrome images (such as cyan,yellow, magenta, or black) may be developed on imaging drum 1010, one ata time, with each monochrome image then being directly transferred inprecise registration to the same print medium, prior to generation ofthe subsequent monochrome image. In this case, the number of transfersto one print medium corresponds to the number of colors used in formingthe composite color image. The necessary level of transfer registrationbetween the monochrome images composing the composite color image on theprint medium is achieved using a high-registration transport unit, notshown. The transport unit, which may comprise a carrier belt or animaging drum, repeatedly and precisely brings the print medium intotouching contact with the dielectric outer surface 1011 of imaging drum1010 for each necessary image transfer.

A monochrome image (such as cyan, yellow, magenta, or black) isdeveloped on dielectric outer surface 1011 of imaging drum 1010 inaccordance with the steps described hereinabove. The toned image is thentransferred from dielectric outer surface 1011 to an intermediatetransfer medium 1018 (belt, drum, etc.). The transfer may be carried outusing mechanical means, electrostatically or through a combinationthereof. Following cleaning by unit 1022 of the dielectric outer surface1011 of imaging drum 1010, subsequent monochrome images are created anddeveloped on dielectric outer surface 1011 of imaging drum 1010 witheach monochrome image sequentially transferred to an intermediatetransfer medium 1018. After all the monochrome images comprising a fullcolor image have been transferred to the intermediate transfer medium, asingle transfer is carried out to the print medium.

Alternately, each single monochrome image may be individuallytransferred from the Intermediate transfer medium 1018 to the printmedium creating the full composite color image on the print medium.Transfer from the intermediate transfer medium 1018 to the print mediummay be carried out using mechanical means, a combination of mechanicaland thermal means, or electrostatically. Using a heat and pressurecombination unifies the steps of transfer and fixing. The materialcomposition of the intermediate transfer medium and its surface energyare selected in accordance with the type of transfer desired.

A monochrome image (such as cyan, yellow, magenta, or black) isdeveloped on dielectric outer surface 1011 of imaging drum 1010 inaccordance with the steps described hereinabove. Subsequent latentimages are superimposed on each previous developed image and thendeveloped. After a composite color image is created on the dielectricouter surface 1011 of imaging drum 1010, a single transfer eitherdirectly to the print medium or alternately, to the intermediatetransfer medium is carried out using any of the techniques mentionedabove, and dielectric outer surface 1011 is cleaned. In order to effectthe superimposition of latent images onto developed images, the tonerused must exhibit dielectric properties in order that the surfaceconductance of the developed image be sufficiently low to prevent chargeleakage and to preserve high resolution. An example of this type oftoner is described in U.S. Pat. No. 3,337,340 to Matkan.

Fixing unit 1020 is operative to fix the transferred image onto theprint medium. The structure of fixing unit 1020 is determined by thespecific toner used in developing unit 1016. When dry toner is used, thetoner is fused and fixed to the print medium using heat and pressurerollers or alternately using flood radiation heating. When some types ofliquid toner are used, the toner may be fixed to the print medium uponspontaneous or induced evaporation of the liquid carrier. Fixing othertypes of liquid toner may require fusing of the toner particles onto theprint medium by heat, pressure or a combination thereof.

When using an intermediate transfer medium as described hereinabove, thefixing of the image onto the print medium may occur simultaneous withthe transfer, i.e. transfixing, thereby eliminating the need for fixingunit 1020.

Cleaning unit 1022 is operative to remove residual toner remaining onthe dielectric outer surface 1011 of imaging drum 1010 after eachtransfer of toned images from imaging drum 1010. The structure of fixingunit 1022 is determined by the specific toner used in the developingunit 1016. When dry toner is used, removal of residual toner istypically carried out using mechanical means, using a scraping blade,rollers, brushes and/or a combinaton which may also comprise a vacuum.

Additionally, the cleaning stage may include a pre-cleaning step wherebydielectric outer surface 1011 of imaging drum 1010 is electrostaticallytreated. When liquid toner is used, the cleaning stage may comprisemechanical or chemical means or a combination thereof. Chemical meansmay include a local rinse with the carrier component used in the liquidtoner or intergradients.

The relative positioning and engagement of any of the subunitssurrounding digital electrostatic imaging drum 1011 may be selected inaccordance with the desired print cycle, internal configuration andspace considerations of a printer/copier. For example, in accordancewith an alternate embodiment of the present invention, latent imagegeneration unit 1012 may be positioned intermediate developing unit 1016and cleaning unit 1022.

Reference is now made to FIG. 24 which shows a structural illustrationof a printing engine apparatus representing an example of a specificembodiment of the present invention. The printing engine apparatus shownin FIG. 24 is characterized by its ability to create a high qualitycomposite color image on the imaging drum surface and to transfer theimage in one step to a final print medium. The apparatus is alsosuitable for monochrome images.

A conventional printing engine, taken from a Konica 90-28 Digital ColorCopy Machine, from which the conventional drum, was removed was employedby the inventor. This printing engine is specifically configured forcolor prints in formats up to standard paper size A3. A digitalelectrostatic imaging drum 1030 of the type described hereinabove andparticularly shown in FIG. 15C and having an outer diameter of 7.2" (182mm) was employed for latent image generation and developing in place ofthe conventional drum which was removed from the printing engine.

A dielectric outer surface 1032 of imaging drum 1030 comprises animaging area and a non-imaging area where the non-imaging area subtendsa 5.5" (140 mm) arc. The specific structure of the inner electronics ofimaging drum 1030 and of the imaging regions of imaging drum 1030 may begenerally as described hereinabove, particularly with reference to FIG.16B.

The printing engine of FIG. 24 further comprises an edge-definedelongate alternating polarity charge source 1034, developer units 1036,1038, 1040, and 1042, a transfer unit 1044, and a cleaning unit 1046.Furthermore, the printing engine may comprise a fixing unit 1047.

During printing, a print medium (not shown), typically paper, is fed tothe printing engine using a feed unit 1048. A toned image, which maycomprise one or many monochrome images, that was developed on outersurface 1032, is transferred to the print medium in accordance with thetechniques described hereinabove. After transfer, the print medium isoutput via a transport unit 1060 to fixing unit 1047, where the tonedimage is fixed to the final print medium.

Alternating polarity charge source 1034, capable of achieving a spatialedge accuracy consistent with the desired resolution, is typically ofthe type described hereinabove or as described in one or more ofapplicant's co-pending patent applications mentioned above.

In the example shown in FIG. 24, each of developer units 1036, 1038,1040 and 1042 typically contains dry dielectric toner in one of theprimary imaging colors yellow, magenta, cyan and black. The toner istypically two-component based such as described in either of U.S. Pat.Nos. 4,746,589 and 4,679,929 both to Haneda et al or any other toneruseful with the Konica 90-28 digital color copy machine and typicallycomprises relatively small carriers having an average carrier particlesize of approximately 50 microns. Average toner particle size isapproximately 10 microns.

Development of a latent image on outer surface 1032 of imaging drum 1030may be carried out using non-contact development techniques. Usingnon-contact development apparatus enables a composite color image to becreated on outer surface 1032 without mixing of color toners which couldadversely affect the quality of the final image.

During each development cycle, one of the four developer units isactivated. AC voltage with a DC bias is applied to a developing roller1037, 1039, 1041 or 1043 in the corresponding activated developer unit1036, 1038, 1040 or 1042. Application of the AC voltage creates a tonercloud adjacent outer surface 1032 by causing toner particles tooscillate in the space between outer surface 1032 and the developingroller of the activated developer unit This results in developingconditions relatively similar to those found in powder clouddevelopment, a technique characterized by the provision of a high numberof grey levels.

An electrostatic attracting field, caused by the net difference betweenthe apparent surface voltage, as defined hereinabove, of each pixellocation of the latent image and a biased developing roller 1037, 1039,1041 or 1043, is generated during development, attracting toner to outersurface 1032 of imaging drum 1030. The amount of toner electrostaticallyadhering to outer surface 1032 of imaging drum 1030 at each pixellocation is a function of the magnitude of the electrostatic attractingfield and determines the optical density of the toner at that location.

Transfer unit 1044 comprises a dielectric transfer belt 1052, a printmedium charging device 1051, a transfer corona unit 1056, and a printmedium neutralizing AC corona unit 1058. After a final toned imagecomprising one or several superimposed monochrome images is developedand toned on outer surface 1032 of imaging drum 1030, transfer from thesurface 1032 to a print medium takes place as follows:

A print medium (not shown) such as plain paper is fed to transfer unit1044 from feed unit 1048. A print medium charging device 1051 thenelectrostatically adheres the print medium to the top side of a transferbelt 1052. Once adhered, the print medium is carried by transfer belt1052 to a transfer location 1055 where the print medium is brought intocontact with surface 1032 of imaging drum 1030. The final toned imagepresent on outer surface 1032 of imaging drum 1030 is thenelectrostatically transferred directly onto the print medium.

During imaging drum revolutions during which no transfer from outersurface 1032 of imaging drum 1030 to a print medium takes place,transfer belt 1052 is moved out of contact with imaging drum 1030.

Transfer of the final toned image from imaging drum 1030 to the printmedium takes place by activating a transfer corona unit 1056 whichbombards the backside of transfer belt 1052 with charges of a polarityopposite to the polarity of the toner particles. This bombardment of thetransfer belt 1052 causes the toner to be attracted to the print medium.

The print medium, bearing the final toned image, is then transported toa print medium neutralizing AC corona unit 1058 where the print mediumis neutralized and detached from the transfer belt 1052. Afterdetachment, the print medium is fed to a transport unit 1060 and is thentransported to fixing unit 1047 where the final toned image is fixed tothe print medium.

Cleaning unit 1046 preferably comprises an adjustable blade 1062,typically made of polyurethane, and a toner disposal roller 1064. Aftertransfer of the final toned image to the print medium at location 1055,cleaning unit 1046 is brought into operative contact with outer surface1032 of imaging drum 1030 to remove residual toner from outer surface1032. The toner is then collected by toner disposal roller 1064 and isremoved to a toner reservoir (not shown) using an Archimedes' screw1066.

The imaging cycle may comprise the following steps:

Digital data representing an image pattern to be generated is seriallyinput to the inner electronics (not shown) of imaging drum 1030 viarotary connectors (not shown). A latent image corresponding to theelectronic signals input to the imaging drum is created on the imagingregion of outer surface 1032 of imaging drum 1030 using edge-definedelongate alternating polarity charge source 1034. In the exampleillustrated in FIG. 24, an A3-size latent image corresponding to onetoner color may be generated during one revolution of imaging drum 1030.

A second revolution of imaging drum 1030, during which one developerunit is activated, tones the latent image, thereby developing amonochrome image on outer surface 1032. For color prints, subsequentlatent image generation and development cycles are carried out, witheach new latent image being superimposed on the developed image. Whenall the monochrome images comprising the composite color image have beendeveloped on outer surface 1032 of imaging drum 1030, a single transferof the composite color image to the print medium is carried out atlocation 1055.

After the composite color image has been transferred to the printmedium, the print medium is transported via transport unit 1060 tofixing unit 1047 where the composite color image is fixed to the printmedium.

Alternately, the imaging cycle may comprise one transfer from imagingdrum 1030 for each individual monochrome image. In this case, transferunit 1044 requires modifications and may include a carrier belt (notshown) or carrier drum (not shown) and other mechanisms which arecapable of handling the print medium and ensuring precise registrationbetween the monochrome images of the composite color image. An exampleof a carrier drum and its associated mechanisms may be found in FIG. 6of U.S. Pat. No. 5,105,280, assigned to Minolta Camera Kabushiki Kaishaof Osaka, Japan. A further example of a carrier drum and its associatedmechanisms may be found in FIG. 5A of U.S. Pat. No. 5,111,302, assignedto Hewlett-Packard of Palo Alto, Calif.

Reference is now made to FIG. 25 which is a schematic illustrabon ofdigital color copier apparatus implemented using the print engine andimaging techniques and apparatus described hereinabove. The digitalcolor copier apparatus preferably is composed of three main subunits: ascanning subunit 1098, a controller 1099 and a printing subunit 1100.

Scanning subunit 1098 scans the images to be copied and creates digitaldata corresponding to the RGB separations of the scanned image. Thesubunit 1098 may comprise standard color or monochrome scanningcomponents including a generally flat scanning support surface 1101 onwhich an original image is placed prior to copying, a light source 1102,scanning optics 1104, including lens system 1105, CCD arrays andassociated optical colorfilters 1106, and electronics 1108 which carryout analog to digital transfer of scanned data.

Controller 1099 receives data from the scanning subunit and carries outimage processing and memory functions and transfer of the scanned datafrom the RGB format used in scanning to the CMYK format necessary forprinting.

Printing subunit 1100 receives image data on a CMYK format and producesa hard copy of the image on a final print medium, typically cut-sheetpaper. The printing subunit 1100 comprises a print engine 1112 which maybe of the type described hereinabove with reference to FIG. 23 and FIG.24. Paper is fed into print engine 1112 from a paper storage tray 1114by a paper feed 1116.

Image data may be input to the controller 1099 from the scanning subunit1098. Alternately, any other source that can provide electronic imagedata, for example a computer or a fax modem, can be used to provideinput data to the printing subunit 1100 via the controller 1099.

It is appreciated that when using the print techniques describedhereinabove to create print copies of a scanned image, printing of animage may be carried out in a line-by-line fashion. Since scanning of animage to create electronic data for printing may also be carried out ina line-by-line fashion, scanning and printing may occur simultaneouslythus allowing conservation of memory resources.

Reference is now made to FIG. 26 which is a schematic illustration ofprint laminator apparatus implemented using a version of the D/E imagingdrum and modified imaging techniques and apparatus. The print laminatorapparatus preferably comprises a D/E imaging drum 1120, which may beconstructed in accordance with techniques and apparatus describedhereinabove, typically having a dielectric outer surface 1121, a latentimage generation unit 1122, a drum surface conditioning unit 1124, adielectric film medium feed unit 1126, a film attachment unit 1128, adeveloping unit 1130, and a film detachment unit 1132.

The print laminator apparatus is characterized in that it createscomposite color images directly on a thin, typically transparentdielectric film, such as PET polyester, which has been attached to outerdielectric surface 1121 of printing drum 1120. Alternatively, the filmmay replace the outer dielectric surface 1121, provided that suitabledielectric connection is maintained with the remainder of the drum.After developing the composite color image, the film may be detachedfrom the outer dielectric surface 1121 of imaging drum 1120 andlaminated to a final substrate, thus creating a laminated print. Printlaminator apparatus further comprises a laminating unit 1134 and mayalso comprise a cutting unit 1136.

A thin dielectric film, preferably having a thickness of 8-12 microns,is fed into the print engine using dielectric film medium feed unit1126. Typically thin dielectric film may be either hazy or glossy.Preferably, the dielectric film is fed into feed unit 1126 in cutsheets. Alternately, dielectric film medium feed unit 1126 may containcutting apparatus so that the dielectric film output to the print engineapparatus is in cut sheet format.

As the dielectric film is being fed, drum surface conditioning unit 1124preconditions outer dielectric surface 1121 of imaging drum 1120.Preconditioning of outer dielectric surface 1121 prepares the outerdielectric surface 1121 for attachment of the dielectric film thereto.Preconditioning and attachment may comprise any steps that enable goodtemporary adhesion between the dielectric film and the outer dielectricsurface 1121 while maintaining dielectric uniformity across thedielectric film.

For example, when a liquid toner is used to develop the latent image,pre-conditioning may be carried out by rinsing and wetting the outerdielectric surface of the imaging drum with a dielectric liquid, such asthe carrier component, typically Isopar, of the liquid toner. In thiscase, film attachment unit 1128 comprises apparatus which presses thedielectric film against the wet outer dielectric surface 1121 with asqueezing motion to remove the air between the outer dielectric surface1121 and the dielectric medium. The necessary level of attachment isachieved by a vacuum that is then created between the two surfaces.

Once attachment of the dielectric film has been carried out, thedielectric film serves as the imaging surface for latent imagegenerating and color development. Following attachment of the dielectricfilm, a color composite image is generated on the film surface,generally in accordance with image generating techniques describedhereinabove, with a color composite image built up on the film fromlatent images which are superimposed on each previous developed imageand then developed.

Since developing does not take place directly on the outer dielectricsurface 1121 of imaging drum 1120, no toner cleaning unit is necessary.

Once the color composite image is fully developed, film detachment unit1132 is activated, thereby removing the dielectric film from the outerdielectric surface 1121 of imaging drum 1120. Film detachment unit 1132may detach the film by attracting and grabbing the leading edge of thedielectric film. Once the film has been detached, it is fed tolaminating unit 1134 by transport apparatus (not shown).

At laminating unit 1134, the thin dielectric film may be laminated to asupport medium 1135 which may be paper, transparencies, etc. Typically,the dielectric film is laminated to the support material such that theside of the film containing the composite color image is placed incontact with the support medium 1135, protecting the image. The supportmedium 1135 may contain adhesives, such as thermoplastics. Alternativelya typically thermoplastic transparent film 1137 may be sandwichedbetween the dielectric film and the support medium 1135 prior to thelamination.

Laminating unit 1134 preferably uses a combination of pressure and heatto adhere the dielectric film to the support medium 1135, thus creatingthe final print. Optional cutting unit 1136 may be used to define theprecise edges of the final print. When cutting unit 1136 is used, highregistration between the dielectric film and the support medium is notnecessary.

It is appreciated that a print laminator apparatus as described hereincould be used to generate high quality glossy or hazy (mafte) continuoustone images on a variety of media including durable transparencies.

Generating and developing the toned image on the dielectric film, whichthen becomes a part of the final print, eliminates the step oftransferring of a toned image and the resulting degradation of the imagequality. Thus, the final print may be of a higher quality than imagesgenerated through ordinary print techniques.

Furthermore, during lamination the dielectric film encapsulates thetoned image. Therefore, the image is protected from environmentalsources and durability of the image is increased.

Reference is now made to FIG. 27 which is a schematic illustration ofprinting apparatus implementing the imaging techniques and apparatusdescribed hereinabove and in applicant's co-pending patent applicationsmentioned hereinabove, to provide very high speed one-sided or duplexprinting. The printing engine apparatus and associated methods may formthe base of a digital color press or alternately of other print deviceswhere high speed, high quality, continuous tone printing is desired.

For duplex printing, the printing engine apparatus typically comprisesfour individual color imaging units, such as cyan, magenta, yellow andblack 1150, 1152, 1154 and 1156 for imaging one side of the print mediumand an additional four color imaging units, such as cyan, magenta,yellow and black, 1158, 1160, 1162 and 1164 for imaging the second sideof the print medium. It is appreciated that the exact number of colorimaging units employed in the printing engine apparatus may be selectedin accordance with the type of printing and the number of colorsdesired. For example, single sided printing may require only four colorunits. Alternately, five color units may be used whereby silver, gold,magnetic or other special-purpose inks or coatings may be added to thefour process colors. All color units in the printing engine apparatusmay be operated simultaneously to achieve high throughput speeds whichare unaffected by the number of colors used or by duplex printing.

Each color imaging unit preferably comprises a D/E imaging drum having adielectric outer surface, a latent image generating unit, a developingunit, and a cleaning unit, all of which may be of the type describedhereinabove with particular reference to FIG. 24.

A duplex color printing image may be generated as follows:

Color separation data representing an image to be printed on one side ofa print medium is input to each of the color units distributed along oneside of the print medium. Simultaneously, color separation datarepresenting a second image to be printed on the opposite side of theprint medium may be input to each of the color units distributed alongthe second side of the print medium. After an image has been transferredto the print medium from one developing unit, the print medium istransported to the next imaging unit to receive a subsequent colorseparation image. This continues until a composite color image has beendeveloped on both sides of the print medium.

The color units are activated in time synchronizabon, with the specifictiming sequence of each latent image generation, development andtransfer step being determined in accordance with the particularconfiguration of the imaging drum.

Latent images preferably are generated on the D/E imaging drum of eachcolor unit using the latent image generation techniques and apparatusdescribed in applicant's co-pending patent applications mentioned above.The development unit then generates a visible monochrome image on theimaging drum. Each developed monochrome image is then transferred to theprint medium in registration. After transfer, any residual tonerremaining on the imaging drum of each color unit is removed by the colorunits respective cleaning unit.

The print medium may be web roll-fed paper. Alternately, a carrier maybe used to transport cut sheets between each of the color units.

After a composite color image has been created on the print medium, theimage may be fixed to the print medium at fixing units 1166 and 1168.Each fixing unit is typically operative to fix the composite color imageto one side of the print medium. For one sided printing, one fixing unitis sufficient. Typically a fixing method which allows for high speedfixing of the image is used. An example of this type of fixing method isnon-contact fixing using a radiant heat source.

Reference is now made to FIG. 28 which is a schematic illustration of analternative printing apparatus implementing the imaging techniques andapparatus described hereinabove and in applicant's co-pending patentapplications mentioned hereinabove, to provide very high speed printing.The printing engine apparatus and associated methods may form the baseof a digital color press or alternately of other print devices wherehigh speed, high quality, continuous tone printing is desired.

The printing engine apparatus typically comprises four individualimaging units, 1170, 1172, 1174 and 1176. Each imaging unit maycorrespond to one of the four process colors, that is cyan, magenta,yellow and black. Alternately, each imaging unit may be multi-color.Imaging units 1170, 1172, 1174 and 1176 may be synchronized to achievehigh throughput speeds.

Imaging units 1170, 1172, 1174 and 1176 preferably comprise a D/Eimaging drum 1178 having a dielectric outer surface 1180, a latent imagegenerating unit 1182, and a development unit 1184, all of which may beof the type described hereinabove with particular reference to FIG. 23and FIG. 24. Development unit 1184 may further comprise a cleaningsub-unit (not shown). Development unit 1184 may comprise single ormulti-color developing subunits.

Each D/E imaging drum 1178 typically comprises imaging regions andnon-imaging regions as described hereinabove with particular referenceto FIG. 15C. Latent image generating unit 1182 is positionedintermediate development unit 1184 and transfer region 1190. Thespecific placement of latent image generating unit 1182 and developmentunit 1184 is selected in accordance with the specific width of thenon-imaging region of D/E imaging drum 1178. In order to achieve a printcycle of D/E imaging drum 1178 using only two rotations of D/E imagingdrum 1178, it is desirable that the distance between transfer region1190 and development unit 1184 span a length that is approximately twicethe width of the non-imaging region.

In accordance with one embodiment of the invention, the diameter of D/Eimaging drum 1178 is 7". This provides an imaging region with an axiallength of 12" suitable for formats up to A3. For larger formats, a D/Eimaging drum with a larger diameter or a longer axial length, or acombination thereof, can be used.

Visible images are created on D/E imaging drum 1178 as follows:

During a first rotation of D/E imaging drum 1178, an electrostatic imageis developed on dielectric outer surface 1180 using latent imagegeneration techniques and apparatus described in applicant's co-pendingpatent applications mentioned above. During a second rotation of D/Eimaging drum 1178, development unit 1184 develops a visible image ondielectric outer surface 1180 of D/E imaging drum 1178. Followingdeveloping, the visible image is transferred from dielectric outersurface 1180 of D/E imaging drum 1178 to an intermediate transfer belt1186 by a combination of electrical and pressure means applied bytransfer element 1192, as is known in the art.

Intermediate transfer belt 1186 is typically a flexible dielectric beltcomprising multiple layers. Typical desirable properties of intermediatetransfer belt 1186 include appropriate dielectric properties,resilience, and surface energy suitable for accepting toner from a drumand releasing toner during transfer to a final print medium. Rollers1194 are operative to transport and rotate intermediate transfer belt1186. After a visible image has been transferred from dielectric outersurface 1180 of D/E imaging drum 1178 to intermediate transfer belt1186, any residual toner remaining on dielectric outer surface 1180 ofD/E imaging drum 1178 may be removed by cleaning sub-unit (not shown).

A composite color print is created as follows:

Color separation data representing an image to be printed on a printmedium is input to imaging units, 1170, 1172, 1174 and 1176 distributedalong intermediate transfer belt 1186.

Operating in time synchronizabon, each imaging unit 1170, 1172, 1174 and1176 creates a toned image and transfers it to intermediate transferbelt 1186 as described hereinabove. Intermediate transfer belt 1186receives toned images and transports said images into sequentaloperative engagement with imaging units 1170, 1172, 1174 and 1176. Tonedimages from each unit are superimposed in precise register on tonedimages created by other imaging units and present on intermediatetransfer belt 1186 thus creating a composite color image thereon.

After a composite color image has been fully built up on intermediatetransfer belt 1186, the built-up image is carried by intermediatetransfer belt 1186 to a fixing drum 1200. Fixing drum 1200 typicallycomprises an inner heating element (not shown) which is operative toheat the outer surface thereof. Alternately, an external radiant heatingelement 1202 may be used. An example of an external radiant heatingelement 1202 may be found in U.S. Pat. No. 3,893,761 to Buchan et al.

Composite color images from intermediate transfer belt 1186 areimpressed on an output print medium as follows:

A print medium which may be cut sheet paper or any other print mediaincluding plastics, stickers, vinyl, etc. is fed over impressioncylinder 1206.

As intermediate transfer belt 1186 rotates over fixing drum 1200, thepigmented toner particles present in built-up images carried byintermediate transfer belt 1186 are heated, bringing said particles to amolten state.

As print medium is transported between intermediate transfer belt 1186and impression cylinder 1206, a combination of the heat from fixing drum1200 and pressure from impression cylinder 1206, causes the moltenparticles to be impressed and fused on output media creating a compositecolor image thereon.

After a composite-color image has been impressed onto a print medium,intermediate transfer belt 1186 is cleaned with cleaning unit 1204.

The exact number of imaging units employed in the printing apparatus maybe selected in accordance with the type of printing, printing speed, andthe number of colors desired. For example, high-speed highlight printingusing one pass-development of two colors as described herein withparticular reference to FIG. 6B-6C, may be carried out by an alternateembodiment of the apparatus comprising two imaging units, each of whichhas two-color development units, preferably with the same two printcolors.

It is appreciated that any suitable paper handling system may be used.Moreover, in accordance with an alternate embodiment of the presentinvention, a paper handling system utilizing a web as the media inputmay be used, in place of cut sheet apparatus. The paper handling systemmay include an inverting assembly to invert the print medium so thatimages may be impressed on a second side thereof, thus providing aduplex printing function.

Reference is now made to FIG. 29 which shows a structural illustrationof a printing apparatus representing a further example of a specificembodiment of the present invention. The printing apparatus shown inFIG. 29 is characterized by its ability to create high speed images foruse in digital offset presses.

Printing apparatus of FIG. 29 comprises drum assemblies 1250 and 1251.Each drum assembly 1250 and 1251 preferably comprises a pair of D/Eimaging drums indicated in drum assembly 1250 with reference numerals1252 and 1254 and indicated in drum assembly 1251 with referencenumerals 1253 and 1255, D/E imagingng drums 1252, 1253, 1254 and 1255have dielectric outer surfaces, latent image generating unit, and adevelopment unit, all of which may be of the type described hereinabovewith particular reference to FIG. 23, FIG. 24 or FIG. 28. Thedevelopment units of D/E imaging drums 1252, 1253, 1254 and 1255 mayfurther comprise cleaning sub-units (not shown). Moreover, developmentunits may comprise single or multi-color developing subunits.

The print cycles of D/E imaging drums 1252 and 1254 comprise tworotations and are as described hereinabove with particular reference toFIG. 28.

Preferably, the two D/E imaging drums of a single drum assembly areoperated in time synchronization. While one D/E imaging drum is in awriting cycle, the second D/E imaging drum is developing an imagewritten thereon and transferring the developed image to an intermediatetransfer drum 1256. Transfer of the visible image from D/E imaging drumto intermediate transfer drum 1256 is effected using a combination ofelectrical and pressure means, as is known in the art From intermediatetransfer drum 1256, images are impressed on a print medium which ispresent on an impression cylinder 1258. Transfer may be carried outusing a combination of heat and pressure, or by any other alternatesuitable means. Cleaning unit 1257 may be used to remove any residualtoner particles remaining on intermediate transfer drum 1256.

Intermediate transfer drum 1256 typically comprises an outer flexiblelayer. Typical desirable properties include appropriate electricalresistivity, resilience, abrasion-resistance and surface energy suitablefor accepting toner particles from a D/E imaging drum through acombination of pressure and electrical means. Intermediate transfer drum1256 further comprises a heating element (not shown) which heats theouter surface of intermediate transfer drum 1256 thereby transformingthe toner particles into a molten state. Images are transferred fromintermediate transfer drum 1256 to a final print medium throughimpression of molten toner particles thereon. An example of a suitableintermediate transfer drum is as described in U.S. Pat. No. 5,335,054 toLanda et al.

After a visible image has been transferred to intermediate transfer drum1256, any residual toner remaining on dielectric outer surface of D/Eimaging drums may be removed by cleaning sub-units (not shown).

It is appreciated that a drum assembly as described herein providesenhanced print speed due to the fact that an image may be impressed ontoa print medium during every rotation of intermediate transfer drum 1256.

A composite color print is created as follows:

Color separation data representing an image to be printed on a printmedium is input to each of the drum assemblies 1250 and 1251.

A print medium which may be cut sheet paper or any other print mediaincluding plastics, stickers, vinyl, etc. is transported from a mediastorage area 1259 to a feed roller 1260. Feed roller 1260 then transfersthe output medium to impression cylinder 1258.

Operating in time synchronization, D/E imaging drums of each drumassembly successively and repeatedly create visible images in accordancewith the techniques described hereinabove and transfer said images to anintermediate transfer drum 1256. From intermediate transfer drum 1256,images are impressed on a print medium present on impression cylinder1258 as described hereinabove.

In accordance with a simplex mode of operation, one set of images,typically comprising one image which was created on each D/E drum, issequentially impressed in register onto a print medium. Subsequently, anoutput medium, bearing the two color image, is transported by transportroller 1262 to a second transport roller 1263. Transport roller 1263carries print medium to transport roller 1264 where it is transported toa second impression cylinder 1266.

An additional set of images is then created by drum assembly 1251 on asecond intermediate transfer drum 1267, which are then impressed onprint medium via impression cylinder 1266 in accordance with thetechniques described herein, thus creating a composite full color imagethereon. Cleaning unit 1268 may be used to remove any residual tonerparticles remaining on second intermediate transfer drum 1267.

In accordance with an alternate embodiment of the present invention,printing may be carried out in duplex mode. In this embodiment, the setof images impressed onto a print medium comprises two single colorimages from each D/E drum. Thus, a full four color image is created on afirst side of output medium at impression cylinder 1256. In this case,each D/E imaging drum will typically have a developing unit that iscapable of multi-color development. After creation of a full four colorimage, output medium is transported by transport roller 1262 totransport roller 1263. Transport roller 1263 is operative to invert theprint medium. Following inversion, transport roller 1264 carries outputmedium to second impression cylinder 1266, where a full color impressionmay be created on a second side thereof.

Typically, after a composite color image has been impressed on one orboth sides of a print medium, the output medium is transported fromimpression cylinder 1266 to a media output assembly 1270. Typically,media output assembly 1270 comprises paper transport rollers 1272 and1274, a chain delivery 1276 and a media output tray 1278. Media outputassembly 1270 may be of the type typically used in offset printers.

The exact number of drum assemblies employed in the printing apparatusmay be selected in accordance with the type of printing, printing speed,and the number of colors desired.

It is appreciated that paper handling components described hereinincluding media storage area 1259, feed rollers 1260, impressioncylinders 1258 and 1256, transport rollers 1262, 1263 and 1264, andmedia output assembly 1270, may be of the type commonly used in offsetprinting machines.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined only by the claims which follow:

We claim:
 1. Imaging apparatus comprising:a dielectric substrate with at least two generally opposite surfaces; a plurality of elongate electrodes underlying a first surface of said dielectric substrate; imaging circuitry for application of voltage signals to said plurality of electrodes; a charge source operative to supply a flow of charges to a second surface of said dielectric substrate and including: an electrostatic shield transversing said plurality of electrodes, said shield having an edge defining a generally linear boundary disposed along said second surface; and wherein, following said supply of charges, said second surface retains a charge image.
 2. Apparatus according to claim 1 and wherein said elongate shield comprises a physical barrier.
 3. Apparatus according to claim 1 and wherein said charge source comprises an AC corona.
 4. Apparatus according to claim 1 and wherein said charge source comprises:a liquid which exhibits electrical conductivity; a liquid applicator which contains said liquid and which is operative to apply said liquid to said second surface of said dielectric substrate; a physical barrier which defines said generally linear boundary; and a termination electrode, in proximity of said physical barrier, which is operative to transfer charges through said liguid to said second surface of said dielectric substrate.
 5. Apparatus according to claim 4 and wherein said applicator is operative to rinse said second surface of said dielectric element with said liquid, thereby also providing a cleaning function.
 6. Apparatus according to claim 5 and wherein said physical barrier comprises a dielectric blade.
 7. Apparatus according to claim 6 and wherein said termination electrode comprises a conductive coating on one surface of said dielectric blade.
 8. Apparatus according to claim 7 and wherein said conductive coating is electrically coupled to a voltage bias source having a predetermined potential.
 9. Apparatus according to claim 1 and further comprising:a developing unit operative to develop said charge image retained on said dielectric substrate, producing a developed image thereon.
 10. Apparatus according to claim 9 and further comprising:a transfer unit operative to transfer said developed image to an output medium.
 11. A method of imaging comprising the steps of:providing a dielectric substrate with at least two generally opposite surfaces; providing a plurality of elongate electrodes underlying a first surface of said dielectric substrate; applying voltage signals to said plurality of electrodes; supplying a flow of charges to a second surface of said dielectric substrate not beyond a generally linear boundary disposed along said second surface of said dielectric substrate and transversing said plurality of elongate electrodes.
 12. A method according to claim 11 and wherein said step of supplying a flow of charges comprises extraction of ions from an ion pool.
 13. A method according to claim 11 and wherein said step of supplying a flow of charges comprises the step of:applying a liquid to said second surface of said dielectric substrate; providing a physical barrier which defines said generally linear boundary; providing a termination electrode in proximity of said physical barrier; and applying a bias voltage to said termination electrode to transfer charges through said liquid to said second surface of said dielectric substrate.
 14. A method according to claim 13 and wherein said step of applying a liquid rinses said second surface of said dielectric element with said liquid, thereby providing a cleaning function.
 15. A method according to claim 11 and further comprising the step of producing a developed image on said dielectric substrate.
 16. A method according to claim 15 and further comprising the step of transfering said developed image to an output medium.
 17. A method according to claim 11 and further comprising the step of displacing said generally linear boundary relative to said dielectric substrate.
 18. Digital color printing apparatus comprising:an intermediate transfer medium; at least one imaging unit, operative to produce a charge image, disposed about said intermediate transfer medium, said imaging unit comprising: an imaging drum having an outer latent image retaining surface; a plurality of elongate electrodes underlying said outer latent image retaining surface; imaging circuitry for application of voltage signals to said plurality of elongate electrodes; a charge source operative to supply a flow of charges to said outer latent image retaining surface not beyond a generally linear boundary disposed along said outer latent image retaining surface of said imaging drum and transversing said plurality of elongate electrodes; and wherein, following said supply of charges, said latent image retaining surface retains a latent charge image; a developing unit operative to apply toner to said outer latent image retaining surface, producing visible images thereon; and a primary transfer element operative to transfer said visible images to the intermediate transfer medium; and a secondary transfer unit operative to transfer said visible images from said intermediate transfer medium to a print medium.
 19. Apparatus according to claim 18 and wherein each of said developing units of said plurality of image printing units utilizes a distinct toner color.
 20. Apparatus according to claim 18 and wherein said plurality of image printing units are operative to provide full process color.
 21. Apparatus according to claim 18 and wherein said intermediate transfer medium comprises a drum.
 22. Apparatus according to claim 18 and wherein said intermediate transfer medium comprises a belt. 